samples: drivers: soc_flash_nrf5 fix missing flash write access

By default CONFIG_MPU_ALLOW_FLASH_WRITE=n so the example must have been
falling on any write to flash.
This patch adds CONFIG_MPU_ALLOW_FLASH_WRITE=y to project configuration.

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