x86_64: update serial driver to support kernel debugging

arch/x86_64/init.c

@@ -78,7 +78,7 @@ void early_console_init(void)
 		vgacon_init();
 	} else if (!strcmp(dest, "ttyS0")) {
 		/* write log messages to serial port */
-		early_serialcon_init(115200);
+		early_serialcon_init();
 	}
 }
 
@@ -109,6 +109,7 @@ int ml_init(uintptr_t arg)
 
 	parse_cmdline(PTR32(mb->cmdline));
 
+	serial_init(115200);
 	early_console_init();
 
 	bootstrap_cpu_init();