mlxsw: reg: Update RAUHTD register with IPv6 support

Update the register so that the active IPv6 neighbours could be dumped
from the device's neighbour table.

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>