net: dsa: rzn1-a5psw: add FDB support

This commits add forwarding database support to the driver. It
implements fdb_add(), fdb_del() and fdb_dump().

Signed-off-by: Clément Léger <clement.leger@bootlin.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>