EDAC, MCE, AMD: Make MC2 decoding per-family

Currently only AMD Family 15h processors have special handling for MC2
errors. Since upcoming Family 16h will also need unique handling, let's
make MC2 handling part of amd_decoder_ops.

Signed-off-by: Jacob Shin <jacob.shin@amd.com>
Signed-off-by: Borislav Petkov <bp@alien8.de>