netdevsim: Allow setting parent node of rate objects

Implement new devlink ops that allow setting rate node as a parent for
devlink port (leaf) or another devlink node through devlink API.
Expose parent names to netdevsim debugfs in read only mode.

Co-developed-by: Vlad Buslov <vladbu@nvidia.com>
Signed-off-by: Vlad Buslov <vladbu@nvidia.com>
Signed-off-by: Dmytro Linkin <dlinkin@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>