amd64_edac: add sys addr to memory controller mapping helpers

Borislav:

- cleanup comments
- cleanup debug calls
- simplify find_mc_by_sys_addr's exit path

Reviewed-by: Mauro Carvalho Chehab <mchehab@redhat.com>
Signed-off-by: Doug Thompson <dougthompson@xmission.com>
Signed-off-by: Borislav Petkov <borislav.petkov@amd.com>