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

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

To fully utilize hierarchical QoS algorithm new attribute 'tx_weight'
needs to be introduced. Weight attribute allows for usage of Weighted
Fair Queuing arbitration scheme among siblings. This arbitration
scheme can be used simultaneously with the strict priority.

Introduce ability to configure tx_weight from devlink userspace
utility. Make the new attribute optional.

Example commands:
$ devlink port function rate add pci/0000:4b:00.0/node_custom \
  tx_weight 50 parent node_0

$ devlink port function rate set pci/0000:4b:00.0/2 tx_weight 20

Signed-off-by: Michal Wilczynski <michal.wilczynski@intel.com>
Reviewed-by: Wojciech Drewek <wojciech.drewek@intel.com>
Signed-off-by: David Ahern <dsahern@kernel.org>