drivers/timer: Rework the nRF RTC driver.

This is a reworked version of the previous RTC driver. The main
changed is related to the handling _timer_idle_exit() on non-RTC
wake-ups. The previous version didn't announce the elapsed time
to the kernel in _timer_idle_exit(). Additionally, the driver now
makes sure never to announce more idle ticks than the kernel asked
for, since the kernel does not handle negative deltas in its timeout
queues.

Change-Id: I312a357a7ce8f0c22adf5153731064b92870e47e
Signed-off-by: Wojciech Bober <wojciech.bober@nordicsemi.no>
Signed-off-by: Øyvind Hovdsveen <oyvind.hovdsveen@nordicsemi.no>
Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>