ARM: at91: USB AT91 gadget registration for module

Registration of at91_udc as a module will enable SoC
related code.

Fix following an idea from Karel Znamenacek.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Karel Znamenacek <karel@ryston.cz>
Acked-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Cc: stable <stable@vger.kernel.org>