dt-bindings: phy: Add DT bindings documentation for NS2 USB DRD PHY

BROADCOM NORTHSTAR2 USB2 (DUAL ROLE DEVICE) PHY

Required properties:

 - compatible: brcm,ns2-drd-phy

 - reg: offset and length of the NS2 PHY related registers.

 - reg-names

   The below registers must be provided.

   icfg - for DRD ICFG configurations

   rst-ctrl - for DRD IDM reset

   crmu-ctrl - for CRMU core vdd, PHY and PHY PLL reset

   usb2-strap - for port over current polarity reversal

 - #phy-cells: Must be 0. No args required.

 - vbus-gpios: vbus gpio binding

 - id-gpios: id gpio binding

Refer to phy/phy-bindings.txt for the generic PHY binding properties

Example:

	usbdrd_phy: phy@66000960 {

			#phy-cells = <0>;

			compatible = "brcm,ns2-drd-phy";

			reg = <0x66000960 0x24>,

			      <0x67012800 0x4>,

			      <0x6501d148 0x4>,

			      <0x664d0700 0x4>;

			reg-names = "icfg", "rst-ctrl",

				    "crmu-ctrl", "usb2-strap";

			id-gpios = <&gpio_g 30 0>;

			vbus-gpios = <&gpio_g 31 0>;

	};