Documentation: dt-bindings: Add documentation for the Meson USB2 PHYs

Add the documentation for the bindings for the Meson8b and GXBB USB2
PHYs.
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Acked-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>

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

+* Amlogic USB2 PHY
+
+Required properties:
+- compatible:	Depending on the platform this should be one of:
+	"amlogic,meson8b-usb2-phy"
+	"amlogic,meson-gxbb-usb2-phy"
+- reg:		The base address and length of the registers
+- #phys-cells:	should be 0 (see phy-bindings.txt in this directory)
+- clocks:	phandle and clock identifier for the phy clocks
+- clock-names:	"usb_general" and "usb"
+
+Optional properties:
+- resets:	reference to the reset controller
+- phy-supply:	see phy-bindings.txt in this directory
+
+
+Example:
+
+usb0_phy: usb_phy@0 {
+	compatible = "amlogic,meson-gxbb-usb2-phy";
+	#phy-cells = <0>;
+	reg = <0x0 0x0 0x0 0x20>;
+	resets = <&reset RESET_USB_OTG>;
+	clocks = <&clkc CLKID_USB>, <&clkc CLKID_USB0>;
+	clock-names = "usb_general", "usb";
+	phy-supply = <&usb_vbus>;
+};