net: dsa: mv88e6xxx: add VLAN Load support

Implement port_pvid_set and port_vlan_add to add new entries in the VLAN
hardware table, and join ports to them.

The patch also implement the STU Get Next and Load Purge operations,
since it is required to have a valid STU entry for at least all VLANs.

Each VLAN has its own forwarding database, with FID num_ports+1 to 4095.

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