drivers: counter: nrfx_timer: Use shutdown task if available

Add a workaround for NRF52 anomaly 78: "High current consumption when
using timer STOP task only". Use the SHUTDOWN task instead.

For consistency, CLEAR the timer timer after STOPPING on devices that
lack the SHUTDOWN task. This also aligns the behavior with
nrfx_timer_disable().

For devices with the SHUTDOWN task, this restores the behavior previous
to e92323f0.

Fixes #87224

Signed-off-by: Marco Widmer <marco.widmer@bytesatwork.ch>