platform: Add Spike initial support

This patch adds initial platform support Spike emulator.

Signed-off-by: James Clarke <jrtc27@jrtc27.com>
Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>

include/sbi_utils/sys/htif.h

@@ -0,0 +1,19 @@
+/*
+ * SPDX-License-Identifier: BSD-3-Clause
+ *
+ * Copyright (c) 2010-2020, The Regents of the University of California
+ * (Regents).  All Rights Reserved.
+ */
+
+#ifndef __SYS_HTIF_H__
+#define __SYS_HTIF_H__
+
+#include <sbi/sbi_types.h>
+
+void htif_putc(char ch);
+
+int htif_getc(void);
+
+int htif_system_down(u32 type);
+
+#endif