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

To fully utilize offload capabilities of Intel 100G card QoS capabilities
new attribute 'tx_priority' needs to be introduced. This attribute allows
for usage of strict priority arbiter among siblings. This arbitration
scheme attempts to schedule nodes based on their priority as long as the
nodes remain within their bandwidth limit.

Introduce new attribute in devlink-rate that will allow for configuration
of strict priority. 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>