net: dsa: lan9303: Add fdb/mdb manipulation

Add functions for managing the lan9303 ALR (Address Logic
Resolution).

Implement DSA methods: port_fdb_add, port_fdb_del, port_mdb_prepare,
port_mdb_add and port_mdb_del.

Since the lan9303 do not offer reading specific ALR entry, the driver
caches all static entries - in a flat table.

Signed-off-by: Egil Hjelmeland <privat@egil-hjelmeland.no>
Reviewed-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>