amd64_edac: Factor in CC6 save area

F15h and later use a portion of DRAM as a CC6 storage area. BIOS
programs D18F1x[17C:140,7C:40] DRAM Base/Limit accordingly by
subtracting the storage area from the DRAM limit setting. However, in
order for edac to consider that part of DRAM too, we need to include it
into the per-node range.

Signed-off-by: Borislav Petkov <borislav.petkov@amd.com>