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>