drivers: timer: nrf_rtc_timer: Optimize z_nrf_rtc_timer_get_ticks

Converting absolute system ticks to RTC ticks is simple. It needs to be
multiplied by CYC_PER_TICK (which by default is 1). Complex algorithm
was used when driver was not tracking current 64 bit tick and function
was returning uint32_t.

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