dt-bindings: phy-pxa-usb: add bindings

This is the PHY chip for USB OTG on PXA platforms.
Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>

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

+Marvell PXA USB PHY
+-------------------
+
+Required properties:
+- compatible: one of: "marvell,mmp2-usb-phy", "marvell,pxa910-usb-phy",
+	"marvell,pxa168-usb-phy",
+- #phy-cells: must be 0
+
+Example:
+	usb-phy: usbphy@d4207000 {
+		compatible = "marvell,mmp2-usb-phy";
+		reg = <0xd4207000 0x40>;
+		#phy-cells = <0>;
+		status = "okay";
+	};
+
+This document explains the device tree binding. For general
+information about PHY subsystem refer to Documentation/phy.txt