mlxsw: spectrum_ipip: Support decoding IPv6 tunnel addresses

To support mirroring to ip6gretap, the SPAN module needs to be able to
decode IPv6 addresses specified at that tunnel.

Extend mlxsw_sp_ipip_netdev_saddr() and mlxsw_sp_ipip_netdev_daddr() to
support IPv6 addresses. To that end, add and publish a support function
mlxsw_sp_ipip_netdev_parms6().

Signed-off-by: Petr Machata <petrm@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>