s390: use generic serial.h header file

There is no serial driver on s390, especially none that relies on a
bogus BASE_BAUD define. Therefore use the generic header file.
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>

arch/s390/include/asm/serial.h

-/* SPDX-License-Identifier: GPL-2.0 */
-#ifndef _ASM_S390_SERIAL_H
-#define _ASM_S390_SERIAL_H
-
-#define BASE_BAUD 0
-
-#endif /* _ASM_S390_SERIAL_H */