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>