risc-v: implement DBCN write byte

The DBCN extension provides a Console Write Byte call.
Implement function sbi_dbcn_write_byte to invoke it.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>

arch/riscv/include/asm/sbi.h

@@ -170,5 +170,6 @@ int sbi_get_mvendorid(long *mvendorid);
 int sbi_get_marchid(long *marchid);
 int sbi_get_mimpid(long *mimpid);
 void sbi_srst_reset(unsigned long type, unsigned long reason);
+int sbi_dbcn_write_byte(unsigned char ch);
 
 #endif

arch/riscv/lib/sbi.c

@@ -204,6 +204,22 @@ void sbi_srst_reset(unsigned long type, unsigned long reason)
 		  0, 0, 0, 0);
 }
 
+/**
+ * sbi_dbcn_write_byte() - write byte to debug console
+ *
+ * @ch:		byte to be written
+ * Return:	SBI error code (SBI_SUCCESS = 0 on success)
+ */
+int sbi_dbcn_write_byte(unsigned char ch)
+{
+	struct sbiret ret;
+
+	ret = sbi_ecall(SBI_EXT_DBCN,
+			SBI_EXT_DBCN_CONSOLE_WRITE_BYTE,
+			ch, 0, 0, 0, 0, 0);
+	return ret.error;
+}
+
 #ifdef CONFIG_SBI_V01
 
 /**