dt-bindings: phy: add DT bindings for UniPhier USB2 PHY driver

Socionext UniPhier USB2 PHY

This describes the devicetree bindings for PHY interface built into

USB2 controller implemented on Socionext UniPhier SoCs.

Pro4 SoC has both USB2 and USB3 host controllers, however, this USB3

controller doesn't include its own High-Speed PHY. This needs to specify

USB2 PHY instead of USB3 HS-PHY.

Required properties:

- compatible: Should contain one of the following:

    "socionext,uniphier-pro4-usb2-phy" - for Pro4 SoC

    "socionext,uniphier-ld11-usb2-phy" - for LD11 SoC

Sub-nodes:

Each PHY should be represented as a sub-node.

Sub-nodes required properties:

- #phy-cells: Should be 0.

- reg: The number of the PHY.

Sub-nodes optional properties:

- vbus-supply: A phandle to the regulator for USB VBUS.

Refer to phy/phy-bindings.txt for the generic PHY binding properties.

Example:

	soc-glue@5f800000 {

		...

		usb-phy {

			compatible = "socionext,uniphier-ld11-usb2-phy";

			usb_phy0: phy@0 {

				reg = <0>;

				#phy-cells = <0>;

			};

			...

		};

	};

	usb@5a800100 {

		compatible = "socionext,uniphier-ehci", "generic-ehci";

		...

		phy-names = "usb";

		phys = <&usb_phy0>;

	};