[S390] tod clock: announce clocksource as perfect

The Time of Day clock is the standard time source for s390. It is
- monotonic
- allows very fast reading
- architecture guarantees at least microsecond stepping
- available as part of the architecture

We should announce the rate of tod as 400 to be in sync with the
description found in clocksource.h:
"400-499:Perfect The ideal clocksource. A must-use where available."

This change will prefer tod over less reliable clock sources.

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