arm: refactor clearing of exception faults to common code

A number of SoCs clear out the Mem/Bus/Usage and Hard Fault exceptions
during init.  Lets refactor that into a common function so we don't have
to keep duplicating it over and over.

Change-Id: Ida908a9092db37447abcf3c9872f36937982f729
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>