dm: console: Check for serial devices properly

With driver model the serial device is often not called "serial". Mark
driver-model stdio devices so that they can be detected and we can look up
the uclass. This is a more reliable way of finding out whether the console
is connected to a serial device or not.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/include/stdio_dev.h b/include/stdio_dev.h
index e4fc8b1..3164fa2 100644
--- a/include/stdio_dev.h
+++ b/include/stdio_dev.h
@@ -16,6 +16,7 @@
 
 #define DEV_FLAGS_INPUT	 0x00000001	/* Device can be used as input	console */
 #define DEV_FLAGS_OUTPUT 0x00000002	/* Device can be used as output console */
+#define DEV_FLAGS_DM     0x00000004	/* Device priv is a struct udevice * */
 
 /* Device information */
 struct stdio_dev {