gpio: dt-bindings: add ibm,ppc4xx-gpio binding

This patch adds binding information for IBM/AMCC/APM GPIO
Controllers of the PowerPC 4XX series and compatible SoCs.

The "PowerPC 405EP Embedded Processor Data Sheet" has the
following to say about the GPIO controllers: "

 - Controller functions and GPIO registers are programmed
   and accessed via memory-mapped OPB bus master accesses

 - All GPIOs are pin-shared with other functions. DCRs control
   whether a particular pin that has GPIO capabilities acts
   as a GPIO or is used for another purpose.

 - Each GPIO outputs is separately programmable to emulate
   an open-drain driver (i.e. drives to zero, threestated if
   output bit is 1)

"
The ppc4xx_gpio.c driver is part of the platform/sysdev drivers
in arch/powerpc/sysdev.

Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/gpio/ibm,ppc4xx-gpio.txt

@@ -0,0 +1,24 @@
+* IBM/AMCC/APM GPIO Controller for PowerPC 4XX series and compatible SoCs
+
+All GPIOs are pin-shared with other functions. DCRs control whether a
+particular pin that has GPIO capabilities acts as a GPIO or is used for
+another purpose. GPIO outputs are separately programmable to emulate
+an open-drain driver.
+
+Required properties:
+	- compatible: must be "ibm,ppc4xx-gpio"
+	- reg: address and length of the register set for the device
+	- #gpio-cells: must be set to 2. The first cell is the pin number
+		and the second cell is used to specify the gpio polarity:
+		0 = active high
+		1 = active low
+	- gpio-controller: marks the device node as a gpio controller.
+
+Example:
+
+GPIO0: gpio@ef600b00 {
+	compatible = "ibm,ppc4xx-gpio";
+	reg = <0xef600b00 0x00000048>;
+	#gpio-cells = <2>;
+	gpio-controller;
+};