mlxsw: reg: Enable IPv6 on router interfaces

Enable IPv6 and IPv6 forwarding on router interfaces (RIFs), so that
they will be able to receive and forward IPv6 traffic.

Signed-off-by: Arkadi Sharshevsky <arkadis@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>