hrtimer: Reverse patch to switch sys_nanosleep to hrtimer

Patch 6ba1b91213e81aa92b5cf7539f7d2a94ff54947c hrtimer: switch
sys_nanosleep to hrtimer breaks dynamic tick. All usleep calls
will fail as the next_timer_interrupt currently does not scan
hrtimer queues. This means the system will be potentially
asleep in the idle mode when the next hrtimer is supposed to
run. Until hrtimer_next_interrupt is implemented, we'll use
the old sys_nanosleep.