s390: select CONFIG_TTY for use of tty in unconditional keyboard driver

The unconditionally built keyboard driver, drivers/s390/char/keyboard.c,
requires CONFIG_TTY, so select it from CONFIG_S390 to prevent a build
error.

Signed-off-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>