In the protocol of IEEE 1394, phy configuration packet is broadcasted to
the bus to configure all PHYs residing on the bus. It includes two
purposes; selecting root node and optimizing gap count.

This commit adds some helper function to serialize/deserialize the
content of phy configuration packet, as well as some KUnit tests for it.

Link: https://lore.kernel.org/r/20240606235133.231543-2-o-takashi@sakamocchi.jpSigned-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>