net: dsa: mv88e6xxx: setup message ports

All interconnectable Marvell switch chips have an ATU Learn2All feature
which allows newly learnt addresses to be spanned on ports marked as
"Message Port".

This commit configures the DSA ports as Message Port. Note that this has
no effect until the Learn2All feature is enabled.

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