net: dsa: hellcreek: Add devlink FDB region

Allow to dump the FDB table via devlink. This is a useful debugging feature.

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