ARM: at91: fix at91x40 build

patch 738a0fd7 "ARM: at91: fix external interrupts in non-DT case"
fixed a run-time error on some at91 platforms but did not apply
the same change to at91x40, which now doesn't build.

This changes at91x40 in the same way that the other platforms
were changed.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>