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/drivers/serial/serial-uclass.c b/drivers/serial/serial-uclass.c
index 9cae9fb..998d372 100644
--- a/drivers/serial/serial-uclass.c
+++ b/drivers/serial/serial-uclass.c
@@ -353,7 +353,7 @@
 	memset(&sdev, '\0', sizeof(sdev));
 
 	strncpy(sdev.name, dev->name, sizeof(sdev.name));
-	sdev.flags = DEV_FLAGS_OUTPUT | DEV_FLAGS_INPUT;
+	sdev.flags = DEV_FLAGS_OUTPUT | DEV_FLAGS_INPUT | DEV_FLAGS_DM;
 	sdev.priv = dev;
 	sdev.putc = serial_stub_putc;
 	sdev.puts = serial_stub_puts;