net: dsa: add support for switchdev VLAN objects

Add new functions in DSA drivers to access hardware VLAN entries through
SWITCHDEV_OBJ_PORT_VLAN objects:

 - port_pvid_get() and vlan_getnext() to dump a VLAN
 - port_vlan_del() to exclude a port from a VLAN
 - port_pvid_set() and port_vlan_add() to join a port to a VLAN

The DSA infrastructure will ensure that each VLAN of the given range
does not already belong to another bridge. If it does, it will fallback
to software VLAN and won't program the hardware.

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