edac.txt: update information about newer Intel CPUs

There's a chapter at edac.rst written by the time Nehalem
support was added. Such information is used not only by the
Nehalem driver (i7core_edac), but by all newer Intel CPU
architectures that are supported by i7core_edac, sb_edac
and sbx_edac drivers.

Update the information to reflect that.

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