dt-bindings: phy: Add documentation for NSP USB3 PHY

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>;

	};