s390/ipl: fix IPIB virtual vs physical address confusion

The pointer to IPL Parameter Information Block is stored
in the absolute lowcore for later use by dump tools. That
pointer is a virtual address, though it should be physical
instead.

Note, this does not fix a real issue, since virtual and
physical addresses are currently the same.

Suggested-by: Heiko Carstens <hca@linux.ibm.com>
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>