dm: core: Provide ofnode_find_subnode_unit()

The ofnode_find_subnode() function currently processes things two
different ways, so the treatment of unit addresses differs depending on
whether OF_LIVE is enabled or not.

Add a new version which uses the ofnode API and add a test to check that
unit addresses can be matched correctly. Leave the old function in place
for the !OF_LIVE case, to avoid a code-size increase, e.g. on
firefly-rk3288

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/include/dm/ofnode.h b/include/dm/ofnode.h
index 8e4c357..1203934 100644
--- a/include/dm/ofnode.h
+++ b/include/dm/ofnode.h
@@ -607,6 +607,18 @@
  */
 ofnode ofnode_find_subnode(ofnode node, const char *subnode_name);
 
+/**
+ * ofnode_find_subnode_unit() - find a named subnode of a parent node
+ *
+ * @node:	valid reference to parent node
+ * @subnode_name: name of subnode to find, including any unit address. If the
+ *	unit address is omitted, any subnode which matches the name (excluding
+ *	any unit address) is returned
+ * Return: reference to subnode (which can be invalid if there is no such
+ * subnode)
+ */
+ofnode ofnode_find_subnode_unit(ofnode node, const char *subnode_name);
+
 #if CONFIG_IS_ENABLED(DM_INLINE_OFNODE)
 #include <asm/global_data.h>