mlxsw: spectrum_router: Associate neighbour table with nexthop instead of group

As explained in the previous patch, nexthop objects can have both IPv4
and IPv6 nexthops in the same group. Therefore, move the neighbour table
to be a property of the nexthop instead of the nexthop group.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>