serial: Allow more than one serial device to be registered

include/drivers/serial.h

@@ -18,9 +18,12 @@
     51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
  */
 
+#include <list.h>
+
 struct serial_dev {
 	//TODO add more functions and attributes here
 	int (*putc)(char);
+	struct dlist_node list;
 };
 
 int serial_register_device(struct serial_dev *sdev);