usb: doc: udc-xilinx: Add devicetree bindings

Add devicetree bindings for Xilinx udc driver.
Signed-off-by: Subbaraya Sundeep Bhatta <sbhatta@xilinx.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>

Documentation/devicetree/bindings/usb/udc-xilinx.txt

+Xilinx USB2 device controller
+
+Required properties:
+- compatible		: Should be "xlnx,usb2-device-4.00.a"
+- reg			: Physical base address and size of the USB2
+			  device registers map.
+- interrupts		: Should contain single irq line of USB2 device
+			  controller
+- xlnx,has-builtin-dma	: if DMA is included
+
+Example:
+ 		axi-usb2-device@42e00000 {
+                        compatible = "xlnx,usb2-device-4.00.a";
+                        interrupts = <0x0 0x39 0x1>;
+                        reg = <0x42e00000 0x10000>;
+                        xlnx,has-builtin-dma;
+                };
+