test: dm: add test item for ofnode_get_child_count()

Add a test item for ofnode_get_child_count()

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Signed-off-by: Frank Wunderlich <frank-w@public-files.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Weijie Gao <weijie.gao@mediatek.com>