[ARM] 4648/1: i.MX/MX1 ensure more complete AITC initialization

The AITC code did not allow to start kernel, if bootloader
manipulates with interrupt level mask. The change ensures,
that NIMASK is initialized into correct state and that
interrupts enable registers are cleared.

Signed-off-by: Pavel Pisa <pisa@cmp.felk.cvut.cz>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>