EDAC, amd64: Define and register UMC error decode function

How we need to decode UMC errors is different from how we decode bus
errors, so let's define a new function for this. We also need a way to
determine the UMC channel since we're not guaranteed that there is a
fixed relation between channel and MCA bank.

Signed-off-by: Yazen Ghannam <Yazen.Ghannam@amd.com>
Cc: Aravind Gopalakrishnan <aravindksg.lkml@gmail.com>
Cc: linux-edac <linux-edac@vger.kernel.org>
Cc: x86-ml <x86@kernel.org>
Link: http://lkml.kernel.org/r/1480359593-80369-1-git-send-email-Yazen.Ghannam@amd.com


[ Fold in decode_synd_reg(), simplify. ]
Signed-off-by: Borislav Petkov <bp@suse.de>