Driver for Broadcom Northstar USB 3.0 PHY

Required properties:

- compatible: one of: "brcm,ns-ax-usb3-phy", "brcm,ns-bx-usb3-phy".
- reg: register mappings for DMP (Device Management Plugin) and ChipCommon B
       MMI.
- reg-names: "dmp" and "ccb-mii"

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:
	usb3-phy {
		compatible = "brcm,ns-ax-usb3-phy";
		reg = <0x18105000 0x1000>, <0x18003000 0x1000>;
		reg-names = "dmp", "ccb-mii";
		#phy-cells = <0>;
	};