net: dsa: mv88e6xxx: add VLAN Purge support

Add support for the VTU Load Purge operation and implement the
port_vlan_del driver function to remove a port from a VLAN entry, and
delete the VLAN if the given port was its last member.

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