usb: gadget: at91_udc: Update DT binding documentation

Three compatible strings have been added to the at91_udc driver.
Update the documentation accordingly.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>

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

@@ -33,7 +33,11 @@ usb1: ehci@00800000 {
 AT91 USB device controller
 
 Required properties:
- - compatible: Should be "atmel,at91rm9200-udc"
+ - compatible: Should be one of the following
+	       "atmel,at91rm9200-udc"
+	       "atmel,at91sam9260-udc"
+	       "atmel,at91sam9261-udc"
+	       "atmel,at91sam9263-udc"
  - reg: Address and length of the register set for the device
  - interrupts: Should contain macb interrupt
  - clocks: Should reference the peripheral and the AHB clocks