ARM: 8447/1: catch pending imprecise abort on unmask

Install a non-faulting handler just before unmasking imprecise aborts
and switch back to the regular one after unmasking is done.

This catches any pending imprecise abort that the firmware/bootloader
may have left behind that would normally crash the kernel at that point.
As there are apparently a lot of bootlaoders out there that do such a
thing it makes sense to handle it in the common startup code.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Tested-by: Tyler Baker <tyler.baker@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>