ppc4xx_edac: convert driver to use the new edac ABI

The legacy edac ABI is going to be removed. Port the driver to use
and benefit from the new API functionality.

Cc: Josh Boyer <jwboyer@gmail.com>
Cc: Jiri Kosina <jkosina@suse.cz>
Cc: Borislav Petkov <borislav.petkov@amd.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>