dm: core: Add function to get child count of ofnode or device

This patch add function used to get the child count of
a ofnode or a device

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

drivers/core/read.c

@@ -352,3 +352,8 @@ fdt_addr_t dev_read_addr_pci(const struct udevice *dev)
 
 	return addr;
 }
+
+int dev_get_child_count(const struct udevice *dev)
+{
+	return ofnode_get_child_count(dev_ofnode(dev));
+}