amdsbwd: update to support SB8xx southbridges

Many thanks to Tino <tinotom@gmail.com> for drawing my attention to
this, for doing a lot of testing and providing great feedback.
Many thanks to AMD for continuing to release public specifications for
their chipsets.

PR: kern/157568
Tested by: Tino <tinotom@gmail.com>
MFC after: 1 week