The calls via RTC API weren't fast enough for the edge case
of minimum counter value, resulting in the timer never
expiring as the counter had already passed the compare value.
This now uses direct register access functions and also
gets the latest counter value in edge case.
This commit exends systimer options for nRF52 arch. It is possible
to use ARM SysTick either for tickless or non-tickless mode. Also,
it is possible to use the RTC peripheral for tickless mode. This
also re-enables support for WFI/WFE sleep if RTC is used, since
this counter continues to run in this mode (in contrast to SysTick).
