drivers: timer: extend nrf_rtc_timer

This commit introduces the following changes:

* nrf_rtc_timer is extended with a capability to handle RTC overflow,
allowing it to operate on absolute RTC ticks, rather than relative
ticks.
* overflow handling is ZLI-proof and relies on the sys clock
handler being executed twice every RTC counter's overflow.
* callbacks are given an absolute RTC tick value as a parameter instead
of CC register's value. The absolute RTC tick value is the RTC counter
value set during CC channel configuration extended to 64 bits.
* in case the timer's target time is in the past or is the current tick,
the timer fires as soon as possible, however still from the RTC's ISR
context.
* in case an active timer is set again with the same target time, it is
not scheduled again - only its event data is updated. Otherwise, the
timer is scheduled as usual.
* a scheduled timer can be aborted.
* system clock functions are now using 64 bit values internally.

Signed-off-by: Andrzej Kuroś <andrzej.kuros@nordicsemi.no>
Signed-off-by: Jedrzej Ciupis <jedrzej.ciupis@nordicsemi.no>
Signed-off-by: Krzysztof Chruscinski <krzysztof.chruscinski@nordicsemi.no>
Signed-off-by: Paweł Kwiek <pawel.kwiek@nordicsemi.no>