MFC: r223960

MFC: r223960

- Current testing shows that (ab)using the JBC performance counter in bus
  cycle mode as timecounter just works fine. My best guess is that a firmware
  update has fixed this, so check at run-time whether it advances and use a
  positive quality if it does. The latter will cause this timecounter to be
  used instead of the tick counter based one, which just sucks for SMP.
- Remove a redundant NULL assignment from the timecounter initialization.

git-svn-id: svn://svn.freebsd.org/base/stable/8@224387 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f