net: dsa: hellcreek: Add devlink VLAN region

Allow to dump the VLAN table via devlink. This especially useful, because the
driver internally leverages VLANs for the port separation. These are not visible
via the bridge utility.

Signed-off-by: Kurt Kanzenbach <kurt@kmk-computers.de>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>