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: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Chris Zankel <chris@zankel.net>