Driver for Broadcom Northstar USB 3.0 PHY

Required properties:

- compatible: one of: "brcm,ns-ax-usb3-phy", "brcm,ns-bx-usb3-phy".
- reg: address of MDIO bus device
- usb3-dmp-syscon: phandle to syscon with DMP (Device Management Plugin)
		   registers
- #phy-cells: must be 0

Initialization of USB 3.0 PHY depends on Northstar version. There are currently
three known series: Ax, Bx and Cx.
Known A0: BCM4707 rev 0
Known B0: BCM4707 rev 4, BCM53573 rev 2
Known B1: BCM4707 rev 6
Known C0: BCM47094 rev 0

Example:
	mdio: mdio@0 {
		reg = <0x0>;
		#size-cells = <1>;
		#address-cells = <0>;

		usb3-phy@10 {
			compatible = "brcm,ns-ax-usb3-phy";
			reg = <0x10>;
			usb3-dmp-syscon = <&usb3_dmp>;
			#phy-cells = <0>;
		};
	};

	usb3_dmp: syscon@18105000 {
		reg = <0x18105000 0x1000>;
	};