dt-bindings: Add Broadcom STB USB PHY binding document

Add DT bindings document for Broadcom STB USB PHYs
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Al Cooper <alcooperx@gmail.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>

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

+Broadcom STB USB PHY
+
+Required properties:
+ - compatible: brcm,brcmstb-usb-phy
+ - reg: two offset and length pairs.
+	The first pair specifies a manditory set of memory mapped
+	registers used for general control of the PHY.
+	The second pair specifies optional registers used by some of
+	the SoCs that support USB 3.x
+ - #phy-cells: Shall be 1 as it expects one argument for setting
+	       the type of the PHY. Possible values are:
+	       - PHY_TYPE_USB2 for USB1.1/2.0 PHY
+	       - PHY_TYPE_USB3 for USB3.x PHY
+
+Optional Properties:
+- clocks : clock phandles.
+- clock-names: String, clock name.
+- brcm,ipp: Boolean, Invert Port Power.
+  Possible values are: 0 (Don't invert), 1 (Invert)
+- brcm,ioc: Boolean, Invert Over Current detection.
+  Possible values are: 0 (Don't invert), 1 (Invert)
+NOTE: one or both of the following two properties must be set
+- brcm,has-xhci: Boolean indicating the phy has an XHCI phy.
+- brcm,has-eohci: Boolean indicating the phy has an EHCI/OHCI phy.
+- dr_mode: String, PHY Device mode.
+  Possible values are: "host", "peripheral ", "drd" or "typec-pd"
+  If this property is not defined, the phy will default to "host" mode.
+
+Example:
+
+usbphy_0: usb-phy@f0470200 {
+	reg = <0xf0470200 0xb8>,
+		<0xf0471940 0x6c0>;
+	compatible = "brcm,brcmstb-usb-phy";
+	#phy-cells = <1>;
+	dr_mode = "host"
+	brcm,ioc = <1>;
+	brcm,ipp = <1>;
+	brcm,has-xhci;
+	brcm,has-eohci;
+	clocks = <&usb20>, <&usb30>;
+	clock-names = "sw_usb", "sw_usb3";
+};