ARM: at91/dt: trivial: fix USB udc compatible string

To please checkpatch and the tiresome reader, add the "atmel," prefix to the
USB udc compatible string.
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Cc: <stable@vger.kernel.org>  #4.0+
Signed-off-by: Kevin Hilman <khilman@linaro.org>

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

@@ -79,9 +79,9 @@ Atmel High-Speed USB device controller
 
 Required properties:
  - compatible: Should be one of the following
-	       "at91sam9rl-udc"
-	       "at91sam9g45-udc"
-	       "sama5d3-udc"
+	       "atmel,at91sam9rl-udc"
+	       "atmel,at91sam9g45-udc"
+	       "atmel,sama5d3-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