devlink: Introduce new attribute 'tx_weight' to devlink-rate

To fully utilize offload capabilities of Intel 100G card QoS capabilities
new attribute 'tx_weight' needs to be introduced. This attribute allows
for usage of Weighted Fair Queuing arbitration scheme among siblings.
This arbitration scheme can be used simultaneously with the strict
priority.

Introduce new attribute in devlink-rate that will allow for configuration
of Weighted Fair Queueing. New attribute is optional.

Signed-off-by: Michal Wilczynski <michal.wilczynski@intel.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>