memory: atmel-ebi: add DT bindings documentation

The EBI (External Bus Interface) is used to access external peripherals
(NOR, SRAM, NAND, and other specific devices like ethernet controllers).
Each device is assigned a CS line and an address range and can have its
own configuration (timings, access mode, bus width, ...).
This driver provides a generic DT binding to configure a device according
to its requirements.
For specific device controllers (like the NAND one) the SMC timings
should be configured by the controller driver through the matrix and smc
syscon regmaps.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>

Documentation/devicetree/bindings/memory-controllers/atmel,ebi.txt

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+* Device tree bindings for Atmel EBI
+
+The External Bus Interface (EBI) controller is a bus where you can connect
+asynchronous (NAND, NOR, SRAM, ....) and synchronous memories (SDR/DDR SDRAMs).
+The EBI provides a glue-less interface to asynchronous memories through the SMC
+(Static Memory Controller).
+
+Required properties:
+
+- compatible:		"atmel,at91sam9260-ebi"
+			"atmel,at91sam9261-ebi"
+			"atmel,at91sam9263-ebi0"
+			"atmel,at91sam9263-ebi1"
+			"atmel,at91sam9rl-ebi"
+			"atmel,at91sam9g45-ebi"
+			"atmel,at91sam9x5-ebi"
+			"atmel,sama5d3-ebi"
+
+- reg:			Contains offset/length value for EBI memory mapping.
+			This property might contain several entries if the EBI
+			memory range is not contiguous
+
+- #address-cells:	Must be 2.
+			The first cell encodes the CS.
+			The second cell encode the offset into the CS memory
+			range.
+
+- #size-cells:		Must be set to 1.
+
+- ranges:		Encodes CS to memory region association.
+
+- clocks:		Clock feeding the EBI controller.
+			See clock-bindings.txt
+
+Children device nodes are representing device connected to the EBI bus.
+
+Required device node properties:
+
+- reg:			Contains the chip-select id, the offset and the length
+			of the memory region requested by the device.
+
+EBI bus configuration will be defined directly in the device subnode.
+
+Optional EBI/SMC properties:
+
+- atmel,smc-bus-width:		width of the asynchronous device's data bus
+				8, 16 or 32.
+				Default to 8 when undefined.
+
+- atmel,smc-byte-access-type	"write" or "select" (see Atmel datasheet).
+				Default to "select" when undefined.
+
+- atmel,smc-read-mode		"nrd" or "ncs".
+				Default to "ncs" when undefined.
+
+- atmel,smc-write-mode		"nwe" or "ncs".
+				Default to "ncs" when undefined.
+
+- atmel,smc-exnw-mode		"disabled", "frozen" or "ready".
+				Default to "disabled" when undefined.
+
+- atmel,smc-page-mode		enable page mode if present. The provided value
+				defines the page size (supported values: 4, 8,
+				16 and 32).
+
+- atmel,smc-tdf-mode:		"normal" or "optimized". When set to
+				"optimized" the data float time is optimized
+				depending on the next device being accessed
+				(next device setup time is subtracted to the
+				current device data float time).
+				Default to "normal" when undefined.
+
+If at least one atmel,smc- property is defined the following SMC timing
+properties become mandatory. In the other hand, if none of the atmel,smc-
+properties are specified, we assume that the EBI bus configuration will be
+handled by the sub-device driver, and none of those properties should be
+defined.
+
+All the timings are expressed in nanoseconds (see Atmel datasheet for a full
+description).
+
+- atmel,smc-ncs-rd-setup-ns
+- atmel,smc-nrd-setup-ns
+- atmel,smc-ncs-wr-setup-ns
+- atmel,smc-nwe-setup-ns
+- atmel,smc-ncs-rd-pulse-ns
+- atmel,smc-nrd-pulse-ns
+- atmel,smc-ncs-wr-pulse-ns
+- atmel,smc-nwe-pulse-ns
+- atmel,smc-nwe-cycle-ns
+- atmel,smc-nrd-cycle-ns
+- atmel,smc-tdf-ns
+
+Example:
+
+	ebi: ebi@10000000 {
+		compatible = "atmel,sama5d3-ebi";
+		#address-cells = <2>;
+		#size-cells = <1>;
+		atmel,smc = <&hsmc>;
+		atmel,matrix = <&matrix>;
+		reg = <0x10000000 0x10000000
+		       0x40000000 0x30000000>;
+		ranges = <0x0 0x0 0x10000000 0x10000000
+			  0x1 0x0 0x40000000 0x10000000
+			  0x2 0x0 0x50000000 0x10000000
+			  0x3 0x0 0x60000000 0x10000000>;
+		clocks = <&mck>;
+
+		pinctrl-names = "default";
+		pinctrl-0 = <&pinctrl_ebi_addr>;
+
+		nor: flash@0,0 {
+			compatible = "cfi-flash";
+			#address-cells = <1>;
+			#size-cells = <1>;
+			reg = <0x0 0x0 0x1000000>;
+			bank-width = <2>;
+
+			atmel,smc-read-mode = "nrd";
+			atmel,smc-write-mode = "nwe";
+			atmel,smc-bus-width = <16>;
+			atmel,smc-ncs-rd-setup-ns = <0>;
+			atmel,smc-ncs-wr-setup-ns = <0>;
+			atmel,smc-nwe-setup-ns = <8>;
+			atmel,smc-nrd-setup-ns = <16>;
+			atmel,smc-ncs-rd-pulse-ns = <84>;
+			atmel,smc-ncs-wr-pulse-ns = <84>;
+			atmel,smc-nrd-pulse-ns = <76>;
+			atmel,smc-nwe-pulse-ns = <76>;
+			atmel,smc-nrd-cycle-ns = <107>;
+			atmel,smc-nwe-cycle-ns = <84>;
+			atmel,smc-tdf-ns = <16>;
+		};
+	};
+