s390/boot: Uncouple virtual and physical kernel offsets

This is a preparatory rework to allow uncoupling virtual
and physical addresses spaces.

Currently __kaslr_offset is the kernel offset in both
physical memory on boot and in virtual memory after DAT
mode is enabled.

Uncouple these offsets and rename the physical address
space variant to __kaslr_offset_phys while keep the name
__kaslr_offset for the offset in virtual address space.

Do not use __kaslr_offset_phys after DAT mode is enabled
just yet, but still make it a persistent boot variable
for later use.

Use __kaslr_offset and __kaslr_offset_phys offsets in
proper contexts and alter handle_relocs() function to
distinguish between the two.

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