pwm: sunxi: document OF bindings

Allwinner sun4i and sun7i SoC PWM controller

Required properties:

  - compatible: should be one of:

    - "allwinner,sun4i-a10-pwm"

    - "allwinner,sun7i-a20-pwm"

  - reg: physical base address and length of the controller's registers

  - #pwm-cells: should be 3. See pwm.txt in this directory for a description of

    the cells format.

  - clocks: From common clock binding, handle to the parent clock.

Example:

	pwm: pwm@01c20e00 {

		compatible = "allwinner,sun7i-a20-pwm";

		reg = <0x01c20e00 0xc>;

		clocks = <&osc24M>;

		#pwm-cells = <3>;

		status = "disabled";

	};