rockchip: rk3399-evb: add stdout-path for the board

The 'stdout-path' is missing after dts sync.
Fixes: 167efc2c ("arm64: dts: rk3399: Sync v5.7-rc1 from Linux")
Signed-off-by: NKever Yang <kever.yang@rock-chips.com>

arch/arm/dts/rk3399-evb-u-boot.dtsi

@@ -8,6 +8,7 @@
 
 / {
 	chosen {
+		stdout-path = "serial2:1500000n8";
 		u-boot,spl-boot-order = "same-as-spl", &sdhci, &sdmmc;
 	};
 };