pwm: rockchip: document device tree bindings

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>;

	};