Documentation: dt: Add Kona PWM binding

Broadcom Kona PWM controller device tree bindings

This controller has 6 channels.

Required Properties :

- compatible: should contain "brcm,kona-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@3e01a000 {

	compatible = "brcm,bcm11351-pwm", "brcm,kona-pwm";

	reg = <0x3e01a000 0xc4>;

	clocks = <&pwm_clk>;

	#pwm-cells = <3>;

};