storage/flash_map: API for get driver belongs to the flash_area.

Introduce API for get driver structure belongs to the flash_area.

Some more complex operation on flash areas might want to be done using
driver directly. It not make sense to wrap every possible flash related
operation by flash_map API.

For instance mcuboot will require this patch.

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