amd64_edac: add functionality to compute the DRAM hole

Borislav:

- cleanup/fix comments, add BKDG refs
- cleanup debug calls

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>