net: dsa: mv88e6xxx: factorize bridge support

Add MV88E6XXX_FLAG_PORTSTATE and MV88E6XXX_FLAG_VLANTABLE flags to
identify switch models with required 802.1D operations.

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