dtoc: Adjust Node to record its parent

We need to be able to search back up the tree for #address-cells and
 #size-cells. Record the parent of each node to make this easier.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Tested-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Tested-by: Kever Yang <kever.yang@rock-chips.com>