mlxsw: reg: Add support for ratr_ipip6_entry_pack()

The RATR register is used to configure the Router Adjacency (next-hop)
Table.

For IP-in-IP entry, underlay destination IPv4 is saved as part of this
register and underlay destination IPv6 is saved by RIPS register and RATR
saves pointer to it.

Add function for setting 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>