netdevsim: Implement devlink rate leafs tx rate support

Implement new devlink ops that allow shared and max tx rate control for
devlink port rate objects (leafs) through devlink API.

Expose rate values of VF ports to netdevsim debugfs.

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>