s390/setup: use physical pointers for memblock_reserve()

memblock_reserve() function accepts physcal address of a memory
block to be reserved, but provided with virtual memory pointers.

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