phy: add Broadcom Kona USB2 PHY DT binding

Add a binding that describes the Broadcom Kona USB2 PHY found
on the BCM281xx family of SoCs.
Signed-off-by: Matt Porter <mporter@linaro.org>
Acked-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>

Documentation/devicetree/bindings/phy/bcm-phy.txt

+BROADCOM KONA USB2 PHY
+
+Required properties:
+ - compatible: brcm,kona-usb2-phy
+ - reg: offset and length of the PHY registers
+ - #phy-cells: must be 0
+Refer to phy/phy-bindings.txt for the generic PHY binding properties
+
+Example:
+
+	usbphy: usb-phy@3f130000 {
+		compatible = "brcm,kona-usb2-phy";
+		reg = <0x3f130000 0x28>;
+		#phy-cells = <0>;
+	};