riscv: dts: add OpenPiton RISC-V board dts support

Previous device tree for OpenPiton emits a warning during compilation.
This commit fixes the previous warning adds dts to the OpenPiton RISC-V
board and added the device tree to MAINTAINER file.

Signed-off-by: Tianrui Wei <tianrui-wei@outlook.com>
Reported-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>