net: dsa: mv88e6xxx: move PVT description in info

Not all Marvell switch chips feature a Cross-chip Port VLAN Table (PVT).

Chips with a PVT use the same implementation, so a new mv88e6xxx_ops
member won't be necessary yet. Add a "pvt" boolean member to the
mv88e6xxx_info structure and kill the obsolete MV88E6XXX_FLAGS_PVT flag.

Add a mv88e6xxx_has_pvt helper to wrap future checks of that condition.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>