s390/time: select CLOCKSOURCE_VALIDATE_LAST_CYCLE

The value returned by read_tod_clock() will overflow on September 17th 2042.
To avoid that system time jumps back select CLOCKSOURCE_VALIDATE_LAST_CYCLE
which enables a sanity check in order to prevent negative "delta" values.

Signed-off-by: Heiko Carstens <hca@linux.ibm.com>