nrf53: fix RTC pretick for RTC rescheduling by other interrupts

It might happen that while some interrupt handler other than for RTC0
or RTC1 (e.g. for RADIO) is executed, the scheduled pretick CC triggers.
This starts pretick pulses due to the loop through IPC. The change
in pretick schedule did not stop the pretick pulses going through IPC
loop, what caused heavy increase in power consumption.

This commit fixes this behavior.
Added also clarifications for Kconfig option `SOC_NRF53_RTC_PRETICK`.

Signed-off-by: Andrzej Kuroś <andrzej.kuros@nordicsemi.no>