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/board/openpiton/riscv64/MAINTAINERS b/board/openpiton/riscv64/MAINTAINERS
new file mode 100644
index 0000000..f91c000
--- /dev/null
+++ b/board/openpiton/riscv64/MAINTAINERS
@@ -0,0 +1,8 @@
+Openpiton BOARD
+M:	Tianrui Wei<tianrui-wei@outlook.com>
+S:	Maintained
+F:	board/openpiton/riscv64/
+F:	include/configs/openpiton-riscv64.h
+F:	configs/openpiton_riscv64_defconfig
+F:	configs/openpiton_riscv64_spl_defconfig
+F:  drivers/mmc/piton_mmc.c