gpio: dt-bindings: Add documentation for Aspeed GPIO controllers

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Acked-by: Joel Stanley <joel@jms.id.au>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

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

+Aspeed GPIO controller Device Tree Bindings
+-------------------------------------------
+
+Required properties:
+- compatible		: Either "aspeed,ast2400-gpio" or "aspeed,ast2500-gpio"
+
+- #gpio-cells 		: Should be two
+			  - First cell is the GPIO line number
+			  - Second cell is used to specify optional
+			    parameters (unused)
+
+- reg			: Address and length of the register set for the device
+- gpio-controller	: Marks the device node as a GPIO controller.
+- interrupts		: Interrupt specifier (see interrupt bindings for
+			  details)
+- interrupt-controller	: Mark the GPIO controller as an interrupt-controller
+
+Optional properties:
+
+- interrupt-parent     : The parent interrupt controller, optional if inherited
+
+The gpio and interrupt properties are further described in their respective
+bindings documentation:
+
+- Documentation/devicetree/bindings/gpio/gpio.txt
+- Documentation/devicetree/bindings/interrupt-controller/interrupts.txt
+
+  Example:
+	gpio@1e780000 {
+		#gpio-cells = <2>;
+		compatible = "aspeed,ast2400-gpio";
+		gpio-controller;
+		interrupts = <20>;
+		reg = <0x1e780000 0x1000>;
+		interrupt-controller;
+	};