ARM: Allow to mask data aborts ...

Sometimes it's useful to test if a memory operation works or
aborts. This adds data_abort_mask() to ignore data aborts and
data_abort_unmask() to enable them again. This is used in the
next step for the 'md' command so that illegal addresses just
show 'xxxxxxxx' instead of crashing the system.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>