dt-bindings: phy: Add Stingray USB PHY binding document

Broadcom Stingray USB PHY

Required properties:

 - compatible : should be one of the listed compatibles

	- "brcm,sr-usb-combo-phy" is combo PHY has two PHYs, one SS and one HS.

	- "brcm,sr-usb-hs-phy" is a single HS PHY.

 - reg: offset and length of the PHY blocks registers

 - #phy-cells:

   - Must be 1 for brcm,sr-usb-combo-phy as it expects one argument to indicate

     the PHY number of two PHYs. 0 for HS PHY and 1 for SS PHY.

   - Must be 0 for brcm,sr-usb-hs-phy.

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

Example:

	usbphy0: usb-phy@0 {

		compatible = "brcm,sr-usb-combo-phy";

		reg = <0x00000000 0x100>;

		#phy-cells = <1>;

	};

	usbphy1: usb-phy@10000 {

		compatible = "brcm,sr-usb-combo-phy";

		reg = <0x00010000 0x100>,

		#phy-cells = <1>;

	};

	usbphy2: usb-phy@20000 {

		compatible = "brcm,sr-usb-hs-phy";

		reg = <0x00020000 0x100>,

		#phy-cells = <0>;

	};