riscv: Implement semihost.h for earlycon semihost driver

Per RISC-V semihosting spec [1], implement semihost.h for the existing
Arm semihosting earlycon driver to work on RISC-V.

Link: https://github.com/riscv/riscv-semihosting-spec/blob/main/riscv-semihosting-spec.adoc [1]
Signed-off-by: Bin Meng <bmeng@tinylab.org>
Tested-by: Sergey Matyukevich <sergey.matyukevich@syntacore.com>
Acked-by: Palmer Dabbelt <palmer@rivosinc.com>
Link: https://lore.kernel.org/r/20221209150437.795918-3-bmeng@tinylab.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

arch/riscv/include/asm/semihost.h

+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Copyright (C) 2022 tinylab.org
+ * Author: Bin Meng <bmeng@tinylab.org>
+ */
+
+#ifndef _RISCV_SEMIHOST_H_
+#define _RISCV_SEMIHOST_H_
+
+struct uart_port;
+
+static inline void smh_putc(struct uart_port *port, unsigned char c)
+{
+	asm volatile("addi    a1, %0, 0\n"
+		     "addi    a0, zero, 3\n"
+		     ".balign 16\n"
+		     ".option push\n"
+		     ".option norvc\n"
+		     "slli    zero, zero, 0x1f\n"
+		     "ebreak\n"
+		     "srai    zero, zero, 0x7\n"
+		     ".option pop\n"
+		     : : "r" (&c) : "a0", "a1", "memory");
+}
+
+#endif /* _RISCV_SEMIHOST_H_ */

drivers/tty/serial/Kconfig

@@ -75,7 +75,7 @@ config SERIAL_AMBA_PL011_CONSOLE
 
 config SERIAL_EARLYCON_ARM_SEMIHOST
 	bool "Early console using ARM semihosting"
-	depends on ARM64 || ARM
+	depends on ARM64 || ARM || RISCV
 	select SERIAL_CORE
 	select SERIAL_CORE_CONSOLE
 	select SERIAL_EARLYCON