usb: gadget: at91_udc: Allocate udc instance

Allocate udc structure instead of relying on the statically declared
object.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>