net: dsa: mv88e6xxx: Add support for bridge port locked mode

Supporting bridge ports in locked mode using the drop on lock
feature in Marvell mv88e6xxx switchcores is described in the
'88E6096/88E6097/88E6097F Datasheet', sections 4.4.6, 4.4.7 and
5.1.2.1 (Drop on Lock).

This feature is implemented here facilitated by the locked port flag.

Signed-off-by: Hans Schultz <schultz.hans+netdev@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>