ARM: at91: fix external interrupts in non-DT case

Management of external interrupts has changed but the
non-DT code has not integrated these changes.
Add a mask to pass external irq specification from SoC
specific code to the at91_aic_init() function.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Acked-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Cc: stable <stable@vger.kernel.org> [v3.6]