[S390] Use CONFIG_GENERIC_TIME and define TOD clock source.

Fix too slow clock by using CONFIG_GENERIC_TIME and adding a
clock source for the s390 time-of-day clock. As added benefit
we get rid of the s390 specific definition of do_gettimeofday
and do_settimeofday.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>