Broadcom USB3 phy binding for northstar plus SoC
The USB3 phy is internal to the SoC and is accessed using mdio interface.

Required mdio bus properties:
- reg: Should be 0x0 for SoC internal USB3 phy
- #address-cells: must be 1
- #size-cells: must be 0

Required USB3 PHY properties:
- compatible: should be "brcm,nsp-usb3-phy"
- reg: USB3 Phy address on SoC internal MDIO bus and it should be 0x10.
- usb3-ctrl-syscon: handler of syscon node defining physical address
  of usb3 control register.
- #phy-cells: must be 0

Required usb3 control properties:
- compatible: should be "brcm,nsp-usb3-ctrl"
- reg: offset and length of the control registers

Example:

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

		usb3_phy: usb-phy@10 {
			compatible = "brcm,nsp-usb3-phy";
			reg = <0x10>;
			usb3-ctrl-syscon = <&usb3_ctrl>;
			#phy-cells = <0>;
			status = "disabled";
		};
	};

	usb3_ctrl: syscon@104408 {
		compatible = "brcm,nsp-usb3-ctrl", "syscon";
		reg = <0x104408 0x3fc>;
	};