fdt: Avoid recursion when traversing tree

A loop over peers does not need recursion which becomes a problem with
hundreds devices.

This was discovered with "-smp 2048,cores=512,threads=4".

Suggested-by: Jordan Niethe <jniethe5@gmail.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>