dt-bindings: phy-mmp3-usb: Add bindings

This is the PHY chip for USB OTG on MMP3 platform.
Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Reviewed-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/phy/phy-mmp3-usb.txt

+Marvell MMP3 USB PHY
+--------------------
+
+Required properties:
+- compatible: must be "marvell,mmp3-usb-phy"
+- #phy-cells: must be 0
+
+Example:
+	usb-phy: usb-phy@d4207000 {
+		compatible = "marvell,mmp3-usb-phy";
+		reg = <0xd4207000 0x40>;
+		#phy-cells = <0>;
+	};