irqchip: xtensa-pic: xtensa-mx: document DT bindings

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

Documentation/devicetree/bindings/interrupt-controller/cdns,xtensa-mx.txt

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+* Xtensa Interrupt Distributor and Programmable Interrupt Controller (MX)
+
+Required properties:
+- compatible: Should be "cdns,xtensa-mx".
+
+Remaining properties have exact same meaning as in Xtensa PIC
+(see cdns,xtensa-pic.txt).
+
+Examples:
+	pic: pic {
+		compatible = "cdns,xtensa-mx";
+		/* one cell: internal irq number,
+		 * two cells: second cell == 0: internal irq number
+		 *            second cell == 1: external irq number
+		 */
+		#interrupt-cells = <2>;
+		interrupt-controller;
+	};

Documentation/devicetree/bindings/interrupt-controller/cdns,xtensa-pic.txt

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+* Xtensa built-in Programmable Interrupt Controller (PIC)
+
+Required properties:
+- compatible: Should be "cdns,xtensa-pic".
+- interrupt-controller: Identifies the node as an interrupt controller.
+- #interrupt-cells: The number of cells to define the interrupts.
+  It may be either 1 or 2.
+  When it's 1, the first cell is the internal IRQ number.
+  When it's 2, the first cell is the IRQ number, and the second cell
+  specifies whether it's internal (0) or external (1).
+  Periferals are usually connected to a fixed external IRQ, but for different
+  core variants it may be mapped to different internal IRQ.
+  IRQ sensitivity and priority are fixed for each core variant and may not be
+  changed at runtime.
+
+Examples:
+	pic: pic {
+		compatible = "cdns,xtensa-pic";
+		/* one cell: internal irq number,
+		 * two cells: second cell == 0: internal irq number
+		 *            second cell == 1: external irq number
+		 */
+		#interrupt-cells = <2>;
+		interrupt-controller;
+	};