Boot: Skip MSPLIM setting when current architecture is not Armv8-M

Add directives around MSPLIM setting to execute MSPLIM setting
only on Armv8-M.
It can enable the booting on Armv6-M and Armv7-M.

Change-Id: I70c0546f3c1ec48ae80e7a5727c61a397846f43f
Signed-off-by: David Hu <david.hu@arm.com>
1 file changed