net: dsa: add MDB notifier

Add two new DSA_NOTIFIER_MDB_ADD and DSA_NOTIFIER_MDB_DEL events to
notify not only a single switch, but all switches of a the fabric when
an MDB entry is added or removed.

For the moment, keep the current behavior and ignore other switches.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>