drivers/timer: Fixing issue in nRF RTC driver when RTC handler is blocked.

Fixes an unlikely issue that could arise if the RTC handler in the nRF RTC
driver was blocked for more than one sys tick interval. This could lead to
_sys_clock_tick_announce() being called with more than one sys tick when the
kernel did not expect it.

Jira: ZEP-1763

Change-ID: I5608fca6f0ac97a17c1ce452c1c5c67696a49a9a
Signed-off-by: Øyvind Hovdsveen <oyvind.hovdsveen@nordicsemi.no>