devlink: Allow to set up parent in devl_rate_leaf_create()

Currently the driver is able to create leaf nodes for the devlink-rate,
but is unable to set parent for them. This wasn't as issue before the
possibility to export hierarchy from the driver. After adding the export
feature, in order for the driver to supply correct hierarchy, it's
necessary for it to be able to supply a parent name to
devl_rate_leaf_create().

Introduce a new parameter 'parent_name' in devl_rate_leaf_create().

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