drivers/flash: add radio synchronization to RRAM

Added synchronization with BLE radio operation to the RRAM flash
driver. The implementation is using framework which is already
provided for nrf52's flash driver.

Additional added resource locking mechanism while driver does writing
which solves mutual exclusion write access problem.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>
Signed-off-by: Jakub Zymelka <jakub.zymelka@nordicsemi.no>