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

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

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