video: Drop DEV_FLAGS_SYSTEM flag

DEV_FLAGS_SYSTEM does not have any actual meaning, hence drop it.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/board/kosagi/novena/novena.c b/board/kosagi/novena/novena.c
index 69f5be3..919133b 100644
--- a/board/kosagi/novena/novena.c
+++ b/board/kosagi/novena/novena.c
@@ -77,7 +77,7 @@
 	int error;
 	struct stdio_dev dev = {
 		.name	= "button",
-		.flags	= DEV_FLAGS_INPUT | DEV_FLAGS_SYSTEM,
+		.flags	= DEV_FLAGS_INPUT,
 		.start	= novena_gpio_button_init,
 		.getc	= novena_gpio_button_getc,
 		.tstc	= novena_gpio_button_tstc,