mlxsw: Add support for IP-in-IP with IPv6 underlay for Spectrum-2 and above

Currently, mlxsw driver supports IP-in-IP only with IPv4 underlay.
Add support for IPv6 underlay for Spectrum-2 and above.

Most of the configurations are same to IPv4, the main difference between
IPv4 and IPv6 is related to saving IP addresses.
IPv6 addresses are saved as part of KVD and the relevant registers hold
pointer to them.
Add API for that as part of ipip_ops, so then only Spectrum-2 and above
will save IPv6 addresses in this way.

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>