storage: flash map: Add flash_area_sectors

The commit adds flash_area_sectors function that allows to get information
on sector/erase page layout by flash_area object pointer instead of
index.
The only difference between flash_area_sectors and flash_area_get_sectors
is that the later calls flash_area_open internally and as such requires
flash map to be compiled in.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>