modules: hal_nordic: align & extend lp_timer platform

This commit aligns nrf_802154_lp_timer_zephyr platform to new API of
nrf_rtc_timer. Moreover, it implements lp_timer synchronization
functions using the new nrf_rtc_timer API.

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