EDAC, AMD: carve out decoding of MCi_STATUS ErrorCode

This is the MCE error code from the MCi_STATUS banks, bits [15:0] which
describe what type of error was encountered: GART TLB, Memory or Bus
error. The semantics of those bits are identical across all MCE banks so
decode those separately, irrespectively of MCE type.

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