storage/flash_map: Macros for accessing fixed-partitions

The commit adds FIXED_PARTITION_* macros that take DTS node,
in contrary to FLASH_AREA_* macros that take label property,
to access properties of fixed-partition compatible DTS partition
definitions.
There is FIXED_PARTITION_NODE macro provided for translating
DTS labels to nodes.

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