dt-bindings: pwm: Add Broadcom iProc PWM controller binding

Broadcom iProc PWM controller device tree bindings

This controller has 4 channels.

Required Properties :

- compatible: must be "brcm,iproc-pwm"

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

- clocks: phandle + clock specifier pair for the external clock

- #pwm-cells: Should be 3. See pwm.txt in this directory for a

  description of the cells format.

Refer to clocks/clock-bindings.txt for generic clock consumer properties.

Example:

pwm: pwm@18031000 {

	compatible = "brcm,iproc-pwm";

	reg = <0x18031000 0x28>;

	clocks = <&osc>;

	#pwm-cells = <3>;

};