edtlib: link child nodes to parent for nodes with child-bindings

The current EDT graph logic only use properties directly under a
specific node to add dependencies. For nodes properties in
child-bindings, this means that the child phandles are only linked by
the child node itself, which does have an ordinal but no corresponding
"sturct device" in the code, causing those dependencies to be silently
ignored by gen_handles.py.

Fix that by adding the recursive logic to visit child bindings when
present, which causes all child node property handles to be linked to
the parent node.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>