net: dsa: mv88e6xxx: enable ATU Learn2All

The ATU Learn2All feature allows newly learnt addresses to be spanned on
ports marked as "Message Port", currently all DSA ports.

This commit enables this feature which is necessary and quite convenient
for multi-chip switch fabrics.

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