Documentation: bindings: add dt documentation for Rockchip usb PHY

This patch adds a binding that describes the Rockchip usb PHYs
found on Rockchip SoCs usb interface.
Signed-off-by: Yunzhi Li <lyz@rock-chips.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>

Documentation/devicetree/bindings/phy/rockchip-usb-phy.txt

+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>;
+	};
+};