Looking up a descendant node based on a path is a common operation. Add a helper function to do so, to avoid loops in the callers. Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com> Reviewed-by: Barnabás Pőcze <barnabas.pocze@ideasonboard.com> Reviewed-by: Isaac Scott <isaac.scott@ideasonboard.com>