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>
4 files changed