devicetree: document the BCM63XX gated clock bindings

Gated Clock Controller Bindings for MIPS based BCM63XX SoCs

Required properties:

- compatible: must be one of:

	 "brcm,bcm3368-clocks"

	 "brcm,bcm6328-clocks"

	 "brcm,bcm6358-clocks"

	 "brcm,bcm6362-clocks"

	 "brcm,bcm6368-clocks"

	 "brcm,bcm63268-clocks"

- reg: Address and length of the register set

- #clock-cells: must be <1>

Example:

clkctl: clock-controller@10000004 {

	compatible = "brcm,bcm6328-clocks";

	reg = <0x10000004 0x4>;

	#clock-cells = <1>;

};