dm: core: Add a comment for DM_FLAG_OF_PLATDATA

This flag is missing a comment. Add one.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/include/dm/device.h b/include/dm/device.h
index d7ad9d6..2177470 100644
--- a/include/dm/device.h
+++ b/include/dm/device.h
@@ -45,6 +45,7 @@
 /* Device name is allocated and should be freed on unbind() */
 #define DM_FLAG_NAME_ALLOCED		(1 << 7)
 
+/* Device has platform data provided by of-platdata */
 #define DM_FLAG_OF_PLATDATA		(1 << 8)
 
 /*