drivers: Add OSD uclass

Some devices offer a text-based OSD (on-screen display) that can be
programmatically controlled (i.e. text displayed on).

Add a uclass to support such devices.

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Mario Six <mario.six@gdsys.cc>
diff --git a/include/dm/uclass-id.h b/include/dm/uclass-id.h
index 7027ea0..15abdce 100644
--- a/include/dm/uclass-id.h
+++ b/include/dm/uclass-id.h
@@ -92,6 +92,7 @@
 	UCLASS_VIDEO,		/* Video or LCD device */
 	UCLASS_VIDEO_BRIDGE,	/* Video bridge, e.g. DisplayPort to LVDS */
 	UCLASS_VIDEO_CONSOLE,	/* Text console driver for video device */
+	UCLASS_VIDEO_OSD,	/* On-screen display */
 	UCLASS_WDT,		/* Watchdot Timer driver */
 
 	UCLASS_COUNT,