riscv: dts: sophgo: add Milk-V Duo board device tree

Milk-V Duo[1] board is an embedded development platform based on the
CV1800B chip. Add minimal device tree files for the development board.

Support basic uart drivers, so supports booting to a basic shell.

Link: https://milkv.io/duo [1]
Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Acked-by: Chen Wang <unicorn_wang@outlook.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>

arch/riscv/boot/dts/sophgo/Makefile

 # SPDX-License-Identifier: GPL-2.0
+dtb-$(CONFIG_ARCH_SOPHGO) += cv1800b-milkv-duo.dtb
 dtb-$(CONFIG_ARCH_SOPHGO) += sg2042-milkv-pioneer.dtb

arch/riscv/boot/dts/sophgo/cv1800b-milkv-duo.dts

+// SPDX-License-Identifier: (GPL-2.0 OR MIT)
+/*
+ * Copyright (C) 2023 Jisheng Zhang <jszhang@kernel.org>
+ */
+
+/dts-v1/;
+
+#include "cv1800b.dtsi"
+
+/ {
+	model = "Milk-V Duo";
+	compatible = "milkv,duo", "sophgo,cv1800b";
+
+	aliases {
+		serial0 = &uart0;
+		serial1 = &uart1;
+		serial2 = &uart2;
+		serial3 = &uart3;
+		serial4 = &uart4;
+	};
+
+	chosen {
+		stdout-path = "serial0:115200n8";
+	};
+
+	memory@80000000 {
+		device_type = "memory";
+		reg = <0x80000000 0x3f40000>;
+	};
+};
+
+&osc {
+	clock-frequency = <25000000>;
+};
+
+&uart0 {
+	status = "okay";
+};