net: dsa: mv88e6xxx: rework FDB add/del operations

Add a mv88e6xxx_atu_entry structure and a low level function for the ATU
Load operation, and provide FDB add and delete wrappers functions.

This implementation handles the eventual trunk mapping. If the related
bit is set, then the ATU data register would contain the trunk ID, and
not the port vector.

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