edac: add a sysfs node to report the maximum location for the system

The userspace tools need to know what's the maximum location on each
system, as it helps to create nice maps showing how the memory was
filled at the system.

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