Histogram generators are single-pad entities that branch as leaf nodes
at any point in the pipeline. Make sure that pipeline traversal and
routing configuration support them correctly.

Support for the actual HGO and HGT operation will come later.
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>