powerpc: nexus code tidy-up

Make a pass at the various nexus implementations, fixing some very minor
style issues, obsolete comments, etc.

Update the top-level comment to be closer to other nexus
implementations.

The method declaration section has become unwieldy in many respects.
Attempt to tame it by:

• Using generated method typedefs
• Grouping methods roughly by category, and then alphabetically.

Reviewed by: jhb
Differential Revision: https://reviews.freebsd.org/D38495

(cherry picked from commit c514686aa06a08a97df051a75da9ea7964a74fb0)