arm64: dts: rockchip: default serial for Firefly-RK3399

The Firefly-RK3399 uses serial2 with 1,500,000 baud by default
for communication in U-Boot and in the vendor provided distros.

So let us set the same default in the Linux kernel.
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>

arch/arm64/boot/dts/rockchip/rk3399-firefly.dts

@@ -49,6 +49,10 @@ / {
 	model = "Firefly-RK3399 Board";
 	compatible = "firefly,firefly-rk3399", "rockchip,rk3399";
 
+	chosen {
+		stdout-path = "serial2:1500000n8";
+	};
+
 	backlight: backlight {
 		compatible = "pwm-backlight";
 		enable-gpios = <&gpio1 RK_PB5 GPIO_ACTIVE_HIGH>;