mlxsw: reg: Add Router IP version Six Register

The RIPS register is used to store IPv6 addresses for use by the NVE and
IP-in-IP.

For IPv6 underlay support, RATR register needs to hold a pointer to the
remote IPv6 address for encapsulation and RTDP register needs to hold a
pointer to the local IPv6 address for decapsulation check.

Add the required register for saving IPv6 addresses.

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>