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

Import device tree from Linux kernel to add basic support for CPU, PLIC,
UART and Timer. The name cv1800b in the filename represent the chip used
on Milk-V Duo board.

Signed-off-by: Kongyang Liu <seashell11234455@gmail.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>
diff --git a/arch/riscv/dts/Makefile b/arch/riscv/dts/Makefile
index b05bb56..17cda48 100644
--- a/arch/riscv/dts/Makefile
+++ b/arch/riscv/dts/Makefile
@@ -2,6 +2,7 @@
 
 dtb-$(CONFIG_TARGET_ANDES_AE350) += ae350_32.dtb ae350_64.dtb
 dtb-$(CONFIG_TARGET_MICROCHIP_ICICLE) += mpfs-icicle-kit.dtb
+dtb-$(CONFIG_TARGET_MILKV_DUO) += cv1800b-milkv-duo.dtb
 dtb-$(CONFIG_TARGET_QEMU_VIRT) += qemu-virt32.dtb qemu-virt64.dtb
 dtb-$(CONFIG_TARGET_OPENPITON_RISCV64) += openpiton-riscv64.dtb
 dtb-$(CONFIG_TARGET_SIFIVE_UNLEASHED) += hifive-unleashed-a00.dtb