test: create dedicated fdt node for ofnode_for_each_prop test

Property count may change in /buttons node, if more button tests added,
and this will break ofnode_for_each_prop.
Add separate node for mentioned test.

Signed-off-by: Dzmitry Sankouski <dsankouski@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>