Introduce `mlx5_esw_devlink_rate_node_parent_set()` to allow assigning
a parent to scheduling nodes.
Implement `mlx5_esw_qos_node_update_parent()` and
`mlx5_esw_qos_node_validate_set_parent()` to enforce constraints on
node reassignment.
Don't allow reassignment of nodes with active rate objects.
Update `esw_qos_node_set_parent()` to handle cases where
the parent is NULL. A NULL parent indicates that the scheduling element
is attached to the root scheduling element, and since only rate nodes
can be connected to the root, this update is now necessary.