boards: nrf5340: use nrf_reset_network_force_off

The function nrf_reset_network_force_off contains workaround for
errata 161 for nRF5340 SoC. This function should be used instead of
manual writing to NRF_RESET->NETWORK.FORCEOFF register.

Signed-off-by: Andrzej Kuroś <andrzej.kuros@nordicsemi.no>