xtensa: ISS: add BASE_BAUD definition to serial.h

This fixes the following build error in allyesconfig:

drivers/tty/serial/8250/8250_early.c: In function 'parse_options':
drivers/tty/serial/8250/8250_early.c:160:18: error: 'BASE_BAUD' undeclared (first use in this function)
Reported-by: NFengguang Wu <fengguang.wu@intel.com>
Signed-off-by: NMax Filippov <jcmvbkbc@gmail.com>
Signed-off-by: NChris Zankel <chris@zankel.net>

arch/xtensa/platforms/iss/include/platform/serial.h

+/*
+ * This file is subject to the terms and conditions of the GNU General Public
+ * License.  See the file "COPYING" in the main directory of this archive
+ * for more details.
+ *
+ * Copyright (C) 2012 Tensilica Inc.
+ */
+
+#ifndef __ASM_XTENSA_ISS_SERIAL_H
+#define __ASM_XTENSA_ISS_SERIAL_H
+
+/* Have no meaning on ISS, but needed for 8250_early.c */
+#define BASE_BAUD 0
+
+#endif /* __ASM_XTENSA_ISS_SERIAL_H */