rockpro64: Enable HDMI output on rockpro64 board

Reference to commit that adds HDMI to other rk3399 boards:
commit 9778edae5576 ("rockchip: Enable HDMI output on rk3399 board w/ HDMI")

Signed-off-by: Andrius Štikonas <andrius@stikonas.eu>
Cc: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Cc: Jagan Teki <jagan@amarulasolutions.com>
Cc: Kever Yang <kever.yang@rock-chips.com>
Tested-by: Mark Kettenis <kettenis@openbsd.org>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
diff --git a/include/configs/rockpro64_rk3399.h b/include/configs/rockpro64_rk3399.h
index 5f52c1e..7a72cf6 100644
--- a/include/configs/rockpro64_rk3399.h
+++ b/include/configs/rockpro64_rk3399.h
@@ -6,6 +6,11 @@
 #ifndef __ROCKPRO64_RK3399_H
 #define __ROCKPRO64_RK3399_H
 
+#define ROCKCHIP_DEVICE_SETTINGS \
+	"stdin=serial,usbkbd\0" \
+	"stdout=serial,vidconsole\0" \
+	"stderr=serial,vidconsole\0"
+
 #include <configs/rk3399_common.h>
 
 #define CONFIG_SYS_MMC_ENV_DEV 0