usb: gadget: at91_udc: Document DT clocks and clock-names property

The at91_udc driver request 2 clocks, and thus need them to be defined in
the device tree.
Document the clocks and clock-names properties so that everybody use the
correct names.

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

@@ -36,6 +36,10 @@ Required properties:
  - compatible: Should be "atmel,at91rm9200-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
+ - clock-names: Should contains two strings
+		"pclk" for the peripheral clock
+		"hclk" for the AHB clock
 
 Optional properties:
  - atmel,vbus-gpio: If present, specifies a gpio that needs to be