drivers/flash/soc_flash_nrf: suspend POFWARN before engage

nRF52840, nRF52810, nRF52811 and nRF52805 are affected by anomaly 242.
This patch introduces workaround for this anomaly as follow:
Power-fail comparator is disabled before any attempt to erase or write.
Either erase or write is not proceed if EVENT_POFWARN is
already asserted.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>