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

Milk-V Pioneer [1] is a developer motherboard based on SG2042
in a standard mATX form factor.

Currently only support booting into console with only uart
enabled, other features will be added soon later.

Link: https://milkv.io/pioneer [1]
Reviewed-by: Guo Ren <guoren@kernel.org>
Acked-by: Chao Wei <chao.wei@sophgo.com>
Signed-off-by: Chen Wang <unicorn_wang@outlook.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>

arch/riscv/boot/dts/Makefile

@@ -4,6 +4,7 @@ subdir-y += canaan
 subdir-y += microchip
 subdir-y += renesas
 subdir-y += sifive
+subdir-y += sophgo
 subdir-y += starfive
 subdir-y += thead
 

arch/riscv/boot/dts/sophgo/Makefile

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

arch/riscv/boot/dts/sophgo/sg2042-milkv-pioneer.dts

+// SPDX-License-Identifier: GPL-2.0 OR MIT
+/*
+ * Copyright (C) 2022 Sophgo Technology Inc. All rights reserved.
+ */
+
+#include "sg2042.dtsi"
+
+/ {
+	model = "Milk-V Pioneer";
+	compatible = "milkv,pioneer", "sophgo,sg2042";
+
+	chosen {
+		stdout-path = "serial0";
+	};
+};
+
+&uart0 {
+	status = "okay";
+};