of/overlay: Do not generate duplicate nodes

During the course of the rewrites a bug sneaked in when dealing
with children nodes of overlays, which ends up duplicating
sub nodes.

Simply remove the duplicate traversal of child nodes to fix.

Signed-off-by: Pantelis Antoniou <pantelis.antoniou@konsulko.com>
Signed-off-by: Grant Likely <grant.likely@linaro.org>