dt/bindings: Document gpio-ingenic

This commit adds documentation for the devicetree bindings of the
gpio-ingenic driver, which handles GPIOs of the Ingenic SoCs
currently supported by the Linux kernel.
Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

Documentation/devicetree/bindings/gpio/ingenic,gpio.txt

+Ingenic jz47xx GPIO controller
+
+That the Ingenic GPIO driver node must be a sub-node of the Ingenic pinctrl
+driver node.
+
+Required properties:
+--------------------
+
+ - compatible: Must contain one of:
+    - "ingenic,jz4740-gpio"
+    - "ingenic,jz4770-gpio"
+    - "ingenic,jz4780-gpio"
+ - reg: The GPIO bank number.
+ - interrupt-controller: Marks the device node as an interrupt controller.
+ - interrupts: Interrupt specifier for the controllers interrupt.
+ - #interrupt-cells: Should be 2. Refer to
+   ../interrupt-controller/interrupts.txt for more details.
+ - gpio-controller: Marks the device node as a GPIO controller.
+ - #gpio-cells: Should be 2. The first cell is the GPIO number and the second
+    cell specifies GPIO flags, as defined in <dt-bindings/gpio/gpio.h>. Only the
+    GPIO_ACTIVE_HIGH and GPIO_ACTIVE_LOW flags are supported.
+ - gpio-ranges: Range of pins managed by the GPIO controller. Refer to
+   'gpio.txt' in this directory for more details.
+
+Example:
+--------
+
+&pinctrl {
+	#address-cells = <1>;
+	#size-cells = <0>;
+
+	gpa: gpio@0 {
+		compatible = "ingenic,jz4740-gpio";
+		reg = <0>;
+
+		gpio-controller;
+		gpio-ranges = <&pinctrl 0 0 32>;
+		#gpio-cells = <2>;
+
+		interrupt-controller;
+		#interrupt-cells = <2>;
+
+		interrupt-parent = <&intc>;
+		interrupts = <28>;
+	};
+};