riscv: sophgo: milkv_duo: initial support added

Add support for Sophgo's Milk-V Duo board, only minimal device tree and
serial console are enabled, and it can boot via vendor first stage
bootloader.

Signed-off-by: Kongyang Liu <seashell11234455@gmail.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>
diff --git a/board/sophgo/milkv_duo/Kconfig b/board/sophgo/milkv_duo/Kconfig
new file mode 100644
index 0000000..2a458f2
--- /dev/null
+++ b/board/sophgo/milkv_duo/Kconfig
@@ -0,0 +1,28 @@
+if TARGET_MILKV_DUO
+
+config SYS_BOARD
+	default "milkv_duo"
+
+config SYS_VENDOR
+	default "sophgo"
+
+config SYS_CPU
+	default "generic"
+
+config SYS_CONFIG_NAME
+	default "milkv_duo"
+
+config TEXT_BASE
+	default 0x80200000
+
+config ENV_SIZE
+	default 0x20000
+
+config ENV_SECT_SIZE
+	default 0x40000
+
+config BOARD_SPECIFIC_OPTIONS
+	def_bool y
+	select GENERIC_RISCV
+
+endif