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/arch/riscv/Kconfig b/arch/riscv/Kconfig
index b3d7fd8..4b0c3df 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -26,6 +26,9 @@
 config TARGET_SIPEED_MAIX
 	bool "Support Sipeed Maix Board"
 
+config TARGET_OPENPITON_RISCV64
+	bool "Support RISC-V cores on OpenPiton SoC"
+
 endchoice
 
 config SYS_ICACHE_OFF
@@ -60,6 +63,7 @@
 source "board/microchip/mpfs_icicle/Kconfig"
 source "board/sifive/unleashed/Kconfig"
 source "board/sifive/unmatched/Kconfig"
+source "board/openpiton/riscv64/Kconfig"
 source "board/sipeed/maix/Kconfig"
 
 # platform-specific options below