drivers: watchdog: nrfx: add synchronization after stop

In order to ensure that watchdog channels are freed in proper
driver state, synchronization in form of simple loop needs
to be added after stopping. In no irq variant, it is already done
on nrfx level. NRFY function can be replaced by NRFX one in
the future.

Signed-off-by: Michał Stasiak <michal.stasiak@nordicsemi.no>