s390/stp: add sysfs file to show scheduled leap seconds

This patch introduces /sys/devices/system/stp/scheduled_leap_seconds,
which will contain either 0,0 if no leap second is scheduled, or
the UTC timestamp + leap second offset.

Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Reviewed-by: Alexander Egorenkov <egorenar@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>