add a module that provides support for DRAM ECC error injection on AMD CPUs

I imagine that the module would be useful only to a very limited number
of developers, so that's my excuse for not writing any documentation.
On a more serious note, please see DRAM Error Injection section of BKDGs
for families 10h - 16h. E.g. section 2.13.3.1 of BKDG for AMD Family 15h
Models 00h-0Fh Processors.

Many thanks to kib for his suggestions and comments.

Discussed with: kib
MFC after: 3 weeks
Differential Revision: https://reviews.freebsd.org/D9824