mlxsw: spectrum_router: Add mlxsw_sp_ipip_ops

Details of individual tunnel types are kept in an array of
mlxsw_sp_ipip_ops objects. Follow-up patches will use the list to
determine whether a constructed RIF should be a loopback, and to decide
whether a next hop references a tunnel.

The list is currently empty, follow-up patches will add support for GRE.

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>