arm64: dts: rockchip: enable internal SPI flash for RockPro64.

The rockpro64 contains a nor-flash chip connected to spi1.
Signed-off-by: Andrius Štikonas <andrius@stikonas.eu>
[a number of cleanups]
Signed-off-by: Heiko Stuebner <heiko@sntech.de>

arch/arm64/boot/dts/rockchip/rk3399-rockpro64.dts

@@ -642,6 +642,16 @@ &sdhci {
 	status = "okay";
 };
 
+&spi1 {
+	status = "okay";
+
+	flash@0 {
+		compatible = "jedec,spi-nor";
+		reg = <0>;
+		spi-max-frequency = <10000000>;
+	};
+};
+
 &tcphy0 {
 	status = "okay";
 };