EDAC, mpc85xx: Use platform_register/unregister_drivers()

These new helpers simplify implementing multi-driver modules and
properly handle failure to register one driver by unregistering all
previously registered drivers.

Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Cc: linux-edac <linux-edac@vger.kernel.org>
Link: http://lkml.kernel.org/r/1449136632-11680-1-git-send-email-thierry.reding@gmail.com
Signed-off-by: Borislav Petkov <bp@suse.de>