devlink: Allow for devlink-rate nodes parent reassignment

Currently it's not possible to reassign the parent of the node using one
command. As the previous commit introduced a way to export entire
hierarchy from the driver, being able to modify and reassign parents
become important. This way user might easily change QoS settings without
interrupting traffic.

Example command:
devlink port function rate set pci/0000:4b:00.0/1 parent node_custom_1

This reassigns leaf node parent to node_custom_1.

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