amd64_edac: fix chip select handling

Different processor families support a different number of chip selects.
Handle this in a family-dependent way with the proper values assigned at
init time (see amd64_set_dct_base_and_mask).

Remove _DCSM_COUNT defines since they're used at one place and originate
from public documentation.

CC: Keith Mannthey <kmannth@us.ibm.com>
Signed-off-by: Borislav Petkov <borislav.petkov@amd.com>