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>

include/dm/ofnode.h

@@ -879,6 +879,14 @@ ofnode ofnode_by_prop_value(ofnode from, const char *propname,
 	     ofnode_valid(node); \
 	     node = ofnode_next_subnode(node))
 
+/**
+ * ofnode_get_child_count() - get the child count of a ofnode
+ *
+ * @node: valid node to get its child count
+ * @return the number of subnodes
+ */
+int ofnode_get_child_count(ofnode parent);
+
 /**
  * ofnode_translate_address() - Translate a device-tree address
  *