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>