doc: release: 3.4: add note about adding FS_LITTLEFS_FMP_DEV

Update release notes 3.4 with the new Flash Map Kconfig option
for LittleFS.

Signed-off-by: Mykola Kvach <mykola_kvach@epam.com>

# Conflicts:
#	doc/releases/release-notes-3.4.rst