i7core_edac: add support for more than one MC socket

Some Nehalem architectures have more than one MC socket. Socket 0 is
located at bus 255.

Currently, it is using up to 2 sockets, but increasing it to a larger
number is just a matter of increasing MAX_SOCKETS definition.

This seems to be required for properly support of Xeon 55xx.

Still needs testing with Xeon 55xx.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>