dm: core: Allow writing to a flat tree with ofnode

In generally it is not permitted to implement an ofnode function only for
flat tree or live tree. Both must be supported. Also the code for
live tree access should be in of_access.c rather than ofnode.c which is
really just for holding the API-conversion code.

Update ofnode_write_prop() accordingly and fix the test so it can work
with flat tree too.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/include/dm/of_access.h b/include/dm/of_access.h
index 078f2ea..5b7821d 100644
--- a/include/dm/of_access.h
+++ b/include/dm/of_access.h
@@ -519,4 +519,16 @@
  */
 struct device_node *of_get_stdout(void);
 
+/**
+ * of_write_prop() - Write a property to the device tree
+ *
+ * @np:		device node to which the property value is to be written
+ * @propname:	name of the property to write
+ * @value:	value of the property
+ * @len:	length of the property in bytes
+ * Returns: 0 if OK, -ve on error
+ */
+int of_write_prop(struct device_node *np, const char *propname, int len,
+		  const void *value);
+
 #endif