dt-bindings: pinctrl: Add bindings for Microsemi Ocelot

Add the documentation for the Microsemi Ocelot pinmuxing and gpio
controller.

Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: linux-gpio@vger.kernel.org
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

Documentation/devicetree/bindings/pinctrl/mscc,ocelot-pinctrl.txt

+Microsemi Ocelot pin controller Device Tree Bindings
+----------------------------------------------------
+
+Required properties:
+ - compatible		: Should be "mscc,ocelot-pinctrl"
+ - reg			: Address and length of the register set for the device
+ - gpio-controller	: Indicates this device is a GPIO controller
+ - #gpio-cells		: Must be 2.
+			  The first cell is the pin number and the
+			  second cell specifies GPIO flags, as defined in
+			  <dt-bindings/gpio/gpio.h>.
+ - gpio-ranges		: Range of pins managed by the GPIO controller.
+
+
+The ocelot-pinctrl driver uses the generic pin multiplexing and generic pin
+configuration documented in pinctrl-bindings.txt.
+
+The following generic properties are supported:
+ - function
+ - pins
+
+Example:
+	gpio: pinctrl@71070034 {
+		compatible = "mscc,ocelot-pinctrl";
+		reg = <0x71070034 0x28>;
+		gpio-controller;
+		#gpio-cells = <2>;
+		gpio-ranges = <&gpio 0 0 22>;
+
+		uart_pins: uart-pins {
+				pins = "GPIO_6", "GPIO_7";
+				function = "uart";
+		};
+
+		uart2_pins: uart2-pins {
+				pins = "GPIO_12", "GPIO_13";
+				function = "uart2";
+		};
+	};