mlxsw: spectrum_router: Add support for double entry RIFs

In Spectrum-1, loopback router interfaces (RIFs) used for IP-in-IP
encapsulation with an IPv6 underlay require two RIF entries and the RIF
index must be even.

Prepare for this change by extending the RIF parameters structure with a
'double_entry' field that indicates if the RIF being created requires
two RIF entries or not. Only set it for RIFs representing ip6gre tunnels
in Spectrum-1.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Amit Cohen <amcohen@nvidia.com>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>