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>
diff --git a/doc/board/index.rst b/doc/board/index.rst
index 747511f..6fc0442 100644
--- a/doc/board/index.rst
+++ b/doc/board/index.rst
@@ -19,6 +19,7 @@
    intel/index
    kontron/index
    microchip/index
+   openpiton/index
    rockchip/index
    sifive/index
    sipeed/index