dt/bindings: Document pinctrl-ingenic

This commit adds documentation for the devicetree bindings of the
pinctrl-ingenic driver, which handles pin configuration and pin
muxing 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/pinctrl/ingenic,pinctrl.txt

+Ingenic jz47xx pin controller
+
+Please refer to pinctrl-bindings.txt in this directory for details of the
+common pinctrl bindings used by client devices, including the meaning of the
+phrase "pin configuration node".
+
+For the jz47xx SoCs, pin control is tightly bound with GPIO ports. All pins may
+be used as GPIOs, multiplexed device functions are configured within the
+GPIO port configuration registers and it is typical to refer to pins using the
+naming scheme "PxN" where x is a character identifying the GPIO port with
+which the pin is associated and N is an integer from 0 to 31 identifying the
+pin within that GPIO port. For example PA0 is the first pin in GPIO port A, and
+PB31 is the last pin in GPIO port B. The jz4740 contains 4 GPIO ports, PA to
+PD, for a total of 128 pins. The jz4780 contains 6 GPIO ports, PA to PF, for a
+total of 192 pins.
+
+
+Required properties:
+--------------------
+
+ - compatible: One of:
+    - "ingenic,jz4740-pinctrl"
+    - "ingenic,jz4770-pinctrl"
+    - "ingenic,jz4780-pinctrl"
+ - reg: Address range of the pinctrl registers.
+
+
+GPIO sub-nodes
+--------------
+
+The pinctrl node can have optional sub-nodes for the Ingenic GPIO driver;
+please refer to ../gpio/ingenic,gpio.txt.
+
+
+Example:
+--------
+
+pinctrl: pin-controller@10010000 {
+	compatible = "ingenic,jz4740-pinctrl";
+	reg = <0x10010000 0x400>;
+};