ARM: dts: keystone: Add usb phy devicetree bindings

Added device tree support for TI's Keystone USB PHY driver and updated the
Documentation with device tree binding information.
Signed-off-by: WingMan Kwok <w-kwok2@ti.com>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>

Documentation/devicetree/bindings/usb/keystone-phy.txt

+TI Keystone USB PHY
+
+Required properties:
+ - compatible: should be "ti,keystone-usbphy".
+ - #address-cells, #size-cells : should be '1' if the device has sub-nodes
+   with 'reg' property.
+ - reg : Address and length of the usb phy control register set.
+
+The main purpose of this PHY driver is to enable the USB PHY reference clock
+gate on the Keystone SOC for both the USB2 and USB3 PHY. Otherwise it is just
+an NOP PHY driver.  Hence this node is referenced as both the usb2 and usb3
+phy node in the USB Glue layer driver node.
+
+usb_phy: usb_phy@2620738 {
+	compatible = "ti,keystone-usbphy";
+	#address-cells = <1>;
+	#size-cells = <1>;
+	reg = <0x2620738 32>;
+	status = "disabled";
+};

arch/arm/boot/dts/keystone.dtsi

@@ -181,5 +181,13 @@ spi2: spi@21000800 {
 			interrupts = <GIC_SPI 300 IRQ_TYPE_EDGE_RISING>;
 			clocks = <&clkspi>;
 		};
+
+		usb_phy: usb_phy@2620738 {
+			compatible = "ti,keystone-usbphy";
+			#address-cells = <1>;
+			#size-cells = <1>;
+			reg = <0x2620738 32>;
+			status = "disabled";
+		};
 	};
 };