mlxsw: spectrum_router: Enable resilient nexthop groups to be programmed

Now that mlxsw supports resilient nexthop groups, allow them to be
programmed after validating that their configuration conforms to the
device's limitations (e.g., number of buckets is within predefined
range).

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>