lib: sbi: add support for firmware features extension

This extension allows the software running in supervisor mode to control
the behavior of various features of the SBI [1]. Implement the support
for such extension.

Link: https://lists.riscv.org/g/tech-prs/message/924 [1]
Signed-off-by: Clément Léger <cleger@rivosinc.com>
Reviewed-by: Anup Patel <anup@brainfault.org>

include/sbi/sbi_fwft.h

@@ -0,0 +1,23 @@
+/*
+ * SPDX-License-Identifier: BSD-2-Clause
+ *
+ * Copyright (c) 2024 Rivos Inc.
+ *
+ * Authors:
+ *   Clément Léger <cleger@rivosinc.com>
+ */
+
+#ifndef __SBI_FW_FEATURE_H__
+#define __SBI_FW_FEATURE_H__
+
+#include <sbi/sbi_ecall_interface.h>
+
+struct sbi_scratch;
+
+int sbi_fwft_set(enum sbi_fwft_feature_t feature, unsigned long value,
+		 unsigned long flags);
+int sbi_fwft_get(enum sbi_fwft_feature_t feature, unsigned long *out_val);
+
+int sbi_fwft_init(struct sbi_scratch *scratch, bool cold_boot);
+
+#endif