rockchip: dts: rk3399: Add 'same-as-spl' for Rock PI 4

Let the board continue boot from the storage device where
it bootup.

Signed-off-by: Andy Yan <andy.yan@rock-chips.com>
Reviewed-by: Kever Yang <Kever.yang@rock-chips.com>
diff --git a/arch/arm/dts/rk3399-rock-pi-4-u-boot.dtsi b/arch/arm/dts/rk3399-rock-pi-4-u-boot.dtsi
index 30f2b5f..f8d8e33 100644
--- a/arch/arm/dts/rk3399-rock-pi-4-u-boot.dtsi
+++ b/arch/arm/dts/rk3399-rock-pi-4-u-boot.dtsi
@@ -6,6 +6,6 @@
 #include "rk3399-u-boot.dtsi"
 / {
 	chosen {
-		u-boot,spl-boot-order = &sdhci, &sdmmc;
+		u-boot,spl-boot-order = "same-as-spl", &sdhci, &sdmmc;
 	};
 };