net: dsa: lantiq: Add Forwarding Database access

This adds functions to add and remove static entries to and from the
forwarding database and dump the full forwarding database.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>