s390/boot: account Real Memory Copy and Lowcore areas

Real Memory Copy and (absolute) Lowcore areas are
not accounted when virtual memory layout is set up.

Fixes: 4df29d2b ("s390/smp: rework absolute lowcore access")
Fixes: 2f0e8aae ("s390/mm: rework memcpy_real() to avoid DAT-off mode")
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>