Add operations for IP-in-IP GRE6. For now the function can_offload()
returns false and the other functions warn as they should never be called.

A later patch will add dedicated operations for Spectrum-2 which will
support IPv6 underlay, so use 'mlxsw_sp1_' prefix for the new operations.
Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>