Previous adjtime() implementation was limited to adjusting system
timer tick period. This commit reimplements the internals to use
a kernel watchdog timer. Platform-independent part of the code now
works also for adjusting hires RTC and tickless timer rate.
User code facing API is unchanged. Architecture code API has changed:
up_adj_timer_period() is replaced by up_adjtime().
Other improvements:
- Support query of remaining adjustment by passing NULL to first
argument of adjtime(). This matches Linux behavior.
- Improve resolution available for architecture driver, previously
limited to 1 microsecond per tick. Now 1 nanosecond per second.
1825f9534c