lib: utils/serial: add semihosting support

We add RISC-V semihosting based serial console for JTAG based early
debugging.

The RISC-V semihosting specification is available at:
https://github.com/riscv/riscv-semihosting-spec/blob/main/riscv-semihosting-spec.adoc

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Signed-off-by: Kautuk Consul <kconsul@ventanamicro.com>
Reviewed-by: Anup Patel <anup@brainfault.org>

include/sbi_utils/serial/semihosting.h

@@ -0,0 +1,47 @@
+/*
+ * SPDX-License-Identifier: BSD-2-Clause
+ *
+ * Copyright (c) 2022 Ventana Micro Systems Inc.
+ *
+ * Authors:
+ *   Anup Patel <apatel@ventanamicro.com>
+ *   Kautuk Consul <kconsul@ventanamicro.com>
+ */
+
+#ifndef __SERIAL_SEMIHOSTING_H__
+#define __SERIAL_SEMIHOSTING_H__
+
+#include <sbi/sbi_types.h>
+
+/**
+ * enum semihosting_open_mode - Numeric file modes for use with semihosting_open()
+ * MODE_READ: 'r'
+ * MODE_BINARY: 'b'
+ * MODE_PLUS: '+'
+ * MODE_WRITE: 'w'
+ * MODE_APPEND: 'a'
+ *
+ * These modes represent the mode string used by fopen(3) in a form which can
+ * be passed to semihosting_open(). These do NOT correspond directly to %O_RDONLY,
+ * %O_CREAT, etc; see fopen(3) for details. In particular, @MODE_PLUS
+ * effectively results in adding %O_RDWR, and @MODE_WRITE will add %O_TRUNC.
+ * For compatibility, @MODE_BINARY should be added when opening non-text files
+ * (such as images).
+ */
+enum semihosting_open_mode {
+	MODE_READ	= 0x0,
+	MODE_BINARY	= 0x1,
+	MODE_PLUS	= 0x2,
+	MODE_WRITE	= 0x4,
+	MODE_APPEND	= 0x8,
+};
+
+#ifdef CONFIG_SERIAL_SEMIHOSTING
+int semihosting_init(void);
+int semihosting_enabled(void);
+#else
+static inline int semihosting_init(void) { return SBI_ENODEV; }
+static inline int semihosting_enabled(void) { return 0; }
+#endif
+
+#endif