net: dsa: mv88e6xxx: add cross-chip bridging

Implement the DSA cross-chip bridging operations by remapping the local
ports an external source port can egress frames to, when this cross-chip
port joins or leaves a bridge.

The PVT is no longer configured with all ones allowing any external
frame to egress any local port. Only DSA and CPU ports, as well as
bridge group members, can egress frames on local ports.

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