amd64_edac: Carve out ECC-related hw settings

This is in preparation for the init path reorganization where we want
only to

1) test whether a particular node supports ECC
2) can it be enabled

and only then do the necessary allocation/initialization. For that,
we need to decouple the ECC settings of the node from the instance's
descriptor.

The should be no functional change introduced by this patch.

Signed-off-by: Borislav Petkov <borislav.petkov@amd.com>