USB: gadget: atmel_usba: add DT support

Allow to compile the driver all the time if AT91 enabled.

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Acked-by: Felipe Balbi <balbi@ti.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Tested-by: Bo Shen <voice.shen@atmel.com>