subsys: storage: flash_map: add API for check flash driver support

Some applications might want to check whether flash_areas binds to
any flash drive in the system. It might be better to do that while
sanity check at application start-up then while regular run process.
Example of such application is the mcuboot.

This patch introduce such API for checking whether device bindings
were resolved properly during system startup.

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