usb: gadget: pxa27x_udc device-tree documentation

Add documentation for device-tree binding of arm PXA 27x udc (usb
device) driver.

Signed-off-by: Robert Jarzmik <[e-mail]>
Cc: [e-mail]
Acked-by: Arnd Bergmann <[e-mail]>
Signed-off-by: Felipe Balbi <[e-mail]>

Documentation/devicetree/bindings/usb/pxa-usb.txt

···
 		marvell,port-mode = <2>; /* PMM_GLOBAL_MODE */
 	};
 
+UDC
+
+Required properties:
+ - compatible: Should be "marvell,pxa270-udc" for USB controllers
+   used in device mode.
+ - reg: usb device MMIO address space
+ - interrupts: single interrupt generated by the UDC IP
+ - clocks: input clock of the UDC IP (see clock-bindings.txt)
+
+Optional properties:
+ - gpios:
+   - gpio activated to control the USB D+ pullup (see gpio.txt)
+
+Example:
+
+		pxa27x_udc: udc@40600000 {
+			compatible = "marvell,pxa270-udc";
+			reg = <0x40600000 0x10000>;
+			interrupts = <11>;
+			clocks = <&pxa2xx_clks 11>;
+			gpios = <&gpio 22 GPIO_ACTIVE_LOW>;
+		};