usb: gadget: at91_udc: Update DT binding documentation

Add sam9x60 binding.

Signed-off-by: Cristian Birsan <cristian.birsan@microchip.com>

Author: cristibirsan

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

@@ -86,6 +86,7 @@ Required properties:
 	       "atmel,at91sam9rl-udc"
 	       "atmel,at91sam9g45-udc"
 	       "atmel,sama5d3-udc"
+	       "microchip,sam9x60-udc"
  - reg: Address and length of the register set for the device
  - interrupts: Should contain usba interrupt
  - clocks: Should reference the peripheral and host clocks