s390/time: improve steering precision

The common timekeeping code steers the clock by adjusting the multiplier
value of the clock. With the current value of 1000 precision is lost
when the clock is steered with a userspace daemon. Increase the multiplier
and the shift values to increase precision.

Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>