mlxsw: spectrum_router: Publish mlxsw_sp_l3proto

The spectrum_ipip module that will be introduced in the follow-up
patches needs to know the data type.

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>