s390/wti: Introduce infrastructure for warning track interrupt

s390/wti: Introduce infrastructure for warning track interrupt

The warning-track interrupt (wti) provides a notification that the
receiving CPU will be pre-empted from its physical CPU within a short
time frame. This time frame is called grace period and depends on the
machine type. Giving up the CPU on time may prevent a task to get stuck
while holding a resource.

Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Reviewed-by: Mete Durlu <meted@linux.ibm.com>
Signed-off-by: Tobias Huschle <huschle@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>