MFC 264019, 264041, 264048, 264049, 264050, 264051

MFC 264019, 264041, 264048, 264049, 264050, 264051

  Add support for event timers whose clock frequency can change while running.

  Apparently all ARM configs build kern_et.c, but only a few of them also
  build kern_clocksource.c, un-break the build by not referencing functions in
  kern_clocksource if NO_EVENTTIMERS is defined.

  Add variable-frequency support to the arm mpcore eventtimer driver.

  mpcore_timer: Disable the timer and clear any pending bit, then setup the
  new counter register values, then restart the timer.  Also re-nest the parens
  properly for casting the result of converting time and frequency to a count.

git-svn-id: svn://svn.freebsd.org/base/stable/10@266347 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f