[nrf fromtree] Add support for subpartitions in flash_map#3350

nordicjm merged 3 commits intonrfconnect:mainnrfconnect/sdk-zephyr:mainfrom

tomchy:bugfix/storage/NCSDK-NONE_Add_support_for_subpartitions_ncstomchy/sdk-zephyr:bugfix/storage/NCSDK-NONE_Add_support_for_subpartitions_ncsCopy head branch name to clipboard