UPSTREAM: 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>
(cherry picked from commit 3e7f7626fd49a9ffba8520a1a073f62929acad63)
Bug: 109890997
Change-Id: I8f444fa6a2027fb76ad095e70a1313c5e89e86ba
Signed-off-by: John Stultz <john.stultz@linaro.org>