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

* 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>;

};