memory hotplug/s390: set phys_device

Implement arch specific arch_get_memory_phys_device function and
initialize phys_device for each memory section.  That way we finally can
tell which piece of memory belongs to which physical device.

This makes s390's /sys/devices/system/memory/memoryX/phys_device display
the correct thing?

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: Dave Hansen <haveblue@us.ibm.com>
Cc: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>