Documentation: bindings: add dt documentation for Rockchip usb PHY

ROCKCHIP USB2 PHY

Required properties:

 - compatible: rockchip,rk3288-usb-phy

 - rockchip,grf : phandle to the syscon managing the "general

   register files"

 - #address-cells: should be 1

 - #size-cells: should be 0

Sub-nodes:

Each PHY should be represented as a sub-node.

Sub-nodes

required properties:

- #phy-cells: should be 0

- reg: PHY configure reg address offset in GRF

		"0x320" - for PHY attach to OTG controller

		"0x334" - for PHY attach to HOST0 controller

		"0x348" - for PHY attach to HOST1 controller

Optional Properties:

- clocks : phandle + clock specifier for the phy clocks

- clock-names: string, clock name, must be "phyclk"

Example:

usbphy: phy {

	compatible = "rockchip,rk3288-usb-phy";

	rockchip,grf = <&grf>;

	#address-cells = <1>;

	#size-cells = <0>;

	usbphy0: usb-phy0 {

		#phy-cells = <0>;

		reg = <0x320>;

	};

};