devicetree: Add Zynq GPIO devicetree bindings documentation

Add gpio-zynq bindings documentation.
Signed-off-by: Harini Katakam <harinik@xilinx.com>
Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

Documentation/devicetree/bindings/gpio/gpio-zynq.txt

+Xilinx Zynq GPIO controller Device Tree Bindings
+-------------------------------------------
+
+Required properties:
+- #gpio-cells 		: Should be two
+			  - First cell is the GPIO line number
+			  - Second cell is used to specify optional
+			    parameters (unused)
+- compatible		: Should be "xlnx,zynq-gpio-1.0"
+- clocks		: Clock specifier (see clock bindings for details)
+- gpio-controller	: Marks the device node as a GPIO controller.
+- interrupts		: Interrupt specifier (see interrupt bindings for
+			  details)
+- interrupt-parent	: Must be core interrupt controller
+- reg			: Address and length of the register set for the device
+
+Example:
+	gpio@e000a000 {
+		#gpio-cells = <2>;
+		compatible = "xlnx,zynq-gpio-1.0";
+		clocks = <&clkc 42>;
+		gpio-controller;
+		interrupt-parent = <&intc>;
+		interrupts = <0 20 4>;
+		reg = <0xe000a000 0x1000>;
+	};