net: sparx5/lan969x: populate netdev of_node

Populate of_node for the port netdevs, to make the individual ports
of_nodes available in sysfs.

Signed-off-by: Robert Marko <robert.marko@sartura.hr>
Link: https://patch.msgid.link/20251110124342.199216-1-robert.marko@sartura.hr
Signed-off-by: Jakub Kicinski <kuba@kernel.org>