board: riscv: add openpiton-riscv64 SoC support

This patch adds openpiton-riscv64 SOC support. In particular, this
board supports a standard bootflow through zsbl->u-boot SPL->
opensbi->u-boot proper->Linux. There are separate defconfigs for
building u-boot SPL and u-boot proper

Signed-off-by: Tianrui Wei <tianrui-wei@outlook.com>
Signed-off-by: Jonathan Balkind <jbalkind@ucsb.edu>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>