edac: Only build sb_edac on 64-bit

The sb_edac driver is marginally useful on a 32-bit kernel, and
currently has 64-bit divide compile errors when building that config.
For now, make this build on only for 64-bit kernels.

Signed-off-by: Josh Boyer <jwboyer@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>