drivers: timer: nrf_rtc: fix TICKLESS=n processing

Some kernel tests use `CONFIG_TICKLESS_KERNEL=n` with
`CONFIG_SYS_CLOCK_TICKS_PER_SEC=1` to detect when a test runs longer
than 1 second.  These tests break if a tick is announced every time a
timeout occurs.  Only announce if the measured duration since the last
tick is at least the duration of a tick.

Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>