DEVICETREE: Add bindings for the ATH79 GPIO controllers

These bindings support the GPIO controllers found on the Qualcomm
Atheros AR7xxx/AR9XXX SoC.
Signed-off-by: Alban Bedel <albeu@free.fr>
Cc: linux-mips@linux-mips.org
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

Documentation/devicetree/bindings/gpio/gpio-ath79.txt

+Binding for Qualcomm Atheros AR7xxx/AR9xxx GPIO controller
+
+Required properties:
+- compatible: has to be "qca,<soctype>-gpio" and one of the following
+  fallbacks:
+  - "qca,ar7100-gpio"
+  - "qca,ar9340-gpio"
+- reg: Base address and size of the controllers memory area
+- gpio-controller : Marks the device node as a GPIO controller.
+- #gpio-cells : Should be two. The first cell is the pin number and the
+  second cell is used to specify optional parameters.
+- ngpios: Should be set to the number of GPIOs available on the SoC.
+
+Optional properties:
+- interrupt-parent: phandle of the parent interrupt controller.
+- interrupts: Interrupt specifier for the controllers interrupt.
+- interrupt-controller : Identifies the node as an interrupt controller
+- #interrupt-cells : Specifies the number of cells needed to encode interrupt
+		     source, should be 2
+
+Please refer to interrupts.txt in this directory for details of the common
+Interrupt Controllers bindings used by client devices.
+
+Example:
+
+	gpio@18040000 {
+		compatible = "qca,ar9132-gpio", "qca,ar7100-gpio";
+		reg = <0x18040000 0x30>;
+		interrupts = <2>;
+
+		ngpios = <22>;
+
+		gpio-controller;
+		#gpio-cells = <2>;
+
+		interrupt-controller;
+		#interrupt-cells = <2>;
+	};