edac: Add debufs nodes to allow doing fake error inject

Sometimes, it is useful to have a mechanism that generates fake
errors, in order to test the EDAC core code, and the userspace
tools.

Provide such mechanism by adding a few debugfs nodes.

Reviewed-by: Aristeu Rozanski <arozansk@redhat.com>
Cc: Doug Thompson <norsk5@yahoo.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>