pwm: Add device tree binding document for IMG PWM DAC

*Imagination Technologies PWM DAC driver

Required properties:

  - compatible: Should be "img,pistachio-pwm"

  - reg: Should contain physical base address and length of pwm registers.

  - clocks: Must contain an entry for each entry in clock-names.

	See ../clock/clock-bindings.txt for details.

  - clock-names: Must include the following entries.

    - pwm: PWM operating clock.

    - sys: PWM system interface clock.

  - #pwm-cells: Should be 2. See pwm.txt in this directory for the

	description of the cells format.

  - img,cr-periph: Must contain a phandle to the peripheral control

	syscon node which contains PWM control registers.

Example:

	pwm: pwm@18101300 {

		compatible = "img,pistachio-pwm";

		reg = <0x18101300 0x100>;

		clocks = <&pwm_clk>, <&system_clk>;

		clock-names = "pwm", "sys";

		#pwm-cells = <2>;

		img,cr-periph = <&cr_periph>;

	};