net: dsa: mv88e6xxx: assign default FDB to ports

Restore per-port FDB. Assign them on setup, allow adding and deleting
addresses into them, and dump them.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>