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

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

To fully utilize hierarchical QoS algorithm new attribute 'tx_priority'
needs to be introduced. Priority 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 ability to configure tx_priority from devlink userspace
utility. Make the new attribute optional.

Example commands:
$ devlink port function rate add pci/0000:4b:00.0/node_custom \
  tx_priority 5 parent node_0
$ devlink port function rate set pci/0000:4b:00.0/2 tx_priority 5

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>