1. nxsched_process_timer: call clock_update_wall_time() under
CONFIG_CLOCK_TIMEKEEPING so that wall time is updated on timer
events during tickless operation.
2. clock_timekeeping_get_wall_time: call clock_update_wall_time()
before sampling the base and counter.
3. clock_timekeeping: allow overriding NTP_MAX_ADJUST with
CONFIG_CLOCK_ADJTIME_SLEWLIMIT_PPM if configured.
4. Use clock_t consistently for counter values in clock_timekeeping.c.
Signed-off-by: Daniel P. Carvalho <danieloak@gmail.com>