net: dsa: add port_vlan_dump routine

Similar to port_fdb_dump, add a port_vlan_dump function to DSA drivers
which gets passed the switchdev VLAN object and callback.

This function, if implemented, takes precedence over the soon legacy
vlan_getnext/port_pvid_get approach.

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