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

Add DT bindings for PHY interface built into USB2 controller
implemented on Socionext UniPhier SoCs.

Signed-off-by: Kunihiko Hayashi <[e-mail]>
Signed-off-by: Kishon Vijay Abraham I <[e-mail]>

Documentation/devicetree/bindings/phy/uniphier-usb2-phy.txt

···
+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>;
+	};