[S390] sclp: avoid 64 bit division

Avoid 64 bit division to fix this compile error on 32 bit:
drivers/s390/char/sclp_cmd.c:711: undefined reference to `__udivdi3'

Also move the whole arch_get_memory_phys_device function to the
memory hotplug related functions.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>