s390/boot: Add vmem debugging support

Introduce boot_debug() calls in vmem code to log page table mappings
including KASAN shadow mappings for improved early boot debugging.

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