drivers: counter: nrf_timer: Add event clear to set_top_value

At present, if you want to set a periodic timer again with
set_top_value, the last triggered event is not cleared. This could
result in an old event being served at the next interrupt activation.

The solution proposed in this patch adds the nrf_timer_event_clear
function into set_top_value to prevent this from happening in the
above case.

Signed-off-by: Flavia Caforio <flavia.caforio@amarulasolutions.com>