mlxsw: Create separate ipip_ops_arr for different ASICs

Currently, there is support for IP-in-IP only with IPv4 underlay for all
supported Spectrum ASICs.
The next patches will add support for IPv6 underlay only for Spectrum-2
and above.

Add infrastructure for splitting IP-in-IP support between different
ASICs - create separate ipip_ops_arr and add ipips_init function to set the
right ops.

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>