amd64_edac: Erratum #637 workaround

F15h CPUs may report a non-DRAM address when reporting an error address
belonging to a CC6 state save area. Add a workaround to detect this
condition and compute the actual DRAM address of the error as documented
in the Revision Guide for AMD Family 15h Models 00h-0Fh Processors.

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