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

Add DT binding document for Stingray USB PHY.
Signed-off-by: Srinath Mannam <srinath.mannam@broadcom.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>

Documentation/devicetree/bindings/phy/brcm,stingray-usb-phy.txt

+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>;
+	};