e752x_edac: provide more info about how DIMMS/ranks are mapped

No funtional changes here. Only the comments got updated.

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