drivers: clock_control: nrf: Reworked calibration to use k_timer

Reworked calibration to not use HW platform which is not available on
some platforms (nrf53) and does not bring any value compared to using
system timer. Switched to use k_timer. Additionally, improved
calibration algorithm to request also LF clock before starting
calibration. This simplifies the algorithm because it does not need
to support disabling low frequency clock which in calibration.

Tests has been updated and simplified. Former tests relied on HW timer
event.

Signed-off-by: Krzysztof Chruscinski <krzysztof.chruscinski@nordicsemi.no>