dt-bindings: phy: meson-gxl-usb2-phy: Add documentation for the Meson GXL USB2 PHY

This adds the DT binding documentation for the USB2 PHY(s) found in the
Meson GXL and GXM SoCs.
Signed-off-by: NMartin Blumenstingl <martin.blumenstingl@googlemail.com>
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NKishon Vijay Abraham I <kishon@ti.com>

Documentation/devicetree/bindings/phy/meson-gxl-usb2-phy.txt

+* Amlogic Meson GXL and GXM USB2 PHY binding
+
+Required properties:
+- compatible:	Should be "amlogic,meson-gxl-usb2-phy"
+- reg:		The base address and length of the registers
+- #phys-cells:	must be 0 (see phy-bindings.txt in this directory)
+
+Optional properties:
+- phy-supply:	see phy-bindings.txt in this directory
+
+
+Example:
+	usb2_phy0: phy@78000 {
+		compatible = "amlogic,meson-gxl-usb2-phy";
+		#phy-cells = <0>;
+		reg = <0x0 0x78000 0x0 0x20>;
+	};