hwdb: add parser grammar for IEEE 1394 unit function list

hwdb: add parser grammar for IEEE 1394 unit function list

In added IEEE 1394 unit function list, I use custom key to detect unit
entries in node context. Although the list is not widely used in the most
of systemd users, I would like to add parser grammar for testing, by
borrowing a bit time in builders.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>