Documentation: bindings: add doc for the Berlin USB PHY

* Marvell Berlin USB PHY

Required properties:

- compatible: "marvell,berlin2-usb-phy" or "marvell,berlin2cd-usb-phy"

- reg: base address and length of the registers

- #phys-cells: should be 0

- resets: reference to the reset controller

Example:

	usb-phy@f774000 {

		compatible = "marvell,berlin2-usb-phy";

		reg = <0xf774000 0x128>;

		#phy-cells = <0>;

		resets = <&chip 0x104 14>;

	};