Documentation: bindings: add doc for the Berlin USB PHY

Document the bindings of the Marvell Berlin USB PHY driver.
Signed-off-by: NAntoine Tenart <antoine.tenart@free-electrons.com>
Signed-off-by: NKishon Vijay Abraham I <kishon@ti.com>

Documentation/devicetree/bindings/phy/berlin-usb-phy.txt

+* 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>;
+	};