drivers: counter: align to NRF_TIMER hal

Align driver with changes introduced in the hal. `nrf_timer_frequency_set`
was changed to `nrf_timer_prescaler_set`, update driver accordingly.

Signed-off-by: Adam Kondraciuk <adam.kondraciuk@nordicsemi.no>
Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>