mlxsw: reg: Add support for ritr_loopback_ipip6_pack()

The RITR register is used to configure the router interface table.

For IP-in-IP, it stores the underlay source IP address for encapsulation
and also the ingress RIF for the underlay lookup.

Add support for IPv6 IP-in-IP configuration.

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>