net: dsa: mv88e6xxx: factorize VTU access

Add a MV88E6XXX_FLAG_VTU flag to indentify switch models with a VLAN
Table Unit.

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