rockchip: rk3288: dts: enable spl-boot-order

We share the same default SPL boot order for all rk3288 boards,
use dts instead of hard code in board file.

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>
diff --git a/arch/arm/dts/rk3288-u-boot.dtsi b/arch/arm/dts/rk3288-u-boot.dtsi
index 4cf75c7..3f00a3b 100644
--- a/arch/arm/dts/rk3288-u-boot.dtsi
+++ b/arch/arm/dts/rk3288-u-boot.dtsi
@@ -3,6 +3,13 @@
  * Copyright (C) 2019 Rockchip Electronics Co., Ltd
  */
 
+/ {
+	chosen {
+		u-boot,spl-boot-order = \
+			"same-as-spl", &emmc, &sdmmc;
+	};
+};
+
 &dmc {
 	u-boot,dm-pre-reloc;
 };