Driver for Broadcom Northstar USB 2.0 PHY

Required properties:
- compatible: brcm,ns-usb2-phy
- reg: iomem address range of DMU (Device Management Unit)
- reg-names: "dmu", the only needed & supported reg right now
- clocks: USB PHY reference clock
- clock-names: "phy-ref-clk", the only needed & supported clock right now

To initialize USB 2.0 PHY driver needs to setup PLL correctly. To do this it
requires passing phandle to the USB PHY reference clock.

Example:
	usb2-phy {
		compatible = "brcm,ns-usb2-phy";
		reg = <0x1800c000 0x1000>;
		reg-names = "dmu";
		#phy-cells = <0>;
		clocks = <&genpll BCM_NSP_GENPLL_USB_PHY_REF_CLK>;
		clock-names = "phy-ref-clk";
	};