When loading and authenticating an image, all parent images must also
be authenticated. The parent images are just certificates that don't
need to be used by any other CPU except the one loading the image, and
so there is no need to flush the cache of memory regions where they
are loaded.

Change-Id: Ice8d6979d1c02eabf1a543281ae1a07d098e4a99
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>