k_cycle_get_32() implementation on nrf series is too slow.

[nordic-krch]

Timestamp function can be used in time critical operations (event logging). In such scenario current implementation is too slow since it can pend on reading RTC registers which are in LFCLK domain and reading is costly.

zephyr/drivers/timer/nrf_rtc_timer.c

Line 521 in 0aedf8d

u32_t _timer_cycle_get_32(void)