scripts: extract_dts_inculdes: Fix reg prop size/addr cell calculation

When we determine the address and size cells to determine how to parse a
reg property, we need to stop at the parent of the node, not at the node
itself.  If we have #address-cells or #size-cells in a node its meant
for the children of that node and not the node itself.

Fixes #4568

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>