net: dsa: bcm_sf2: Implement FDB operations

Add support for the FDB add, delete, and dump operations. The add and
delete operations are implemented using directed ARL operations using
the specified MAC address and consist in a read operation, write and
readback operation.

The dump operation consists in using the ARL search and software
filtering entries which are not for the desired port.

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