[S390] pfault: delay register of pfault interrupt

Use an early init call to initialize pfault. That way it is possible to
use the register_external_interrupt() instead of the early variant.
No need to enable pfault any earlier since it has only effect if user
space processes are running.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>