drivers/flash/soc_flash_nrf: introduce synchronization api usage

Rework ticker synchronization using newly introduced
radio synchronization API.
In kconfig synchronization using ll ticker become choice
option.

If CONFIG_SOC_FLASH_NRF_PARTIAL_ERASE is enabled the erase
timing is changed so intervals become similar to slots duration.
Previously interval was always ~90 ms, which looks like it was kept
so disproportional by oversight while the partial erase was
introduced.

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