pwm: rockchip: document device tree bindings

This adds binding documentation for Rockchip SoC PWM driver.
Signed-off-by: NBeniamino Galvani <b.galvani@gmail.com>
Signed-off-by: NThierry Reding <thierry.reding@gmail.com>

Documentation/devicetree/bindings/pwm/pwm-rockchip.txt

+Rockchip PWM controller
+
+Required properties:
+ - compatible: should be "rockchip,rk2928-pwm"
+ - reg: physical base address and length of the controller's registers
+ - clocks: phandle and clock specifier of the PWM reference clock
+ - #pwm-cells: should be 2. See pwm.txt in this directory for a
+   description of the cell format.
+
+Example:
+
+	pwm0: pwm@20030000 {
+		compatible = "rockchip,rk2928-pwm";
+		reg = <0x20030000 0x10>;
+		clocks = <&cru PCLK_PWM01>;
+		#pwm-cells = <2>;
+	};