riscv: nexus code tidy-up

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

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 abe3309e71b94527e937f034dd81ca7d6afdeb67)