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.

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.