s390/dump: Add firmware sysfs attribute for dump area size

Dump tools from s390-tools such as zipl need to know the correct dump area
size of the machine they run on in order to be able to create valid
standalone dumper images. Therefore, allow it to be obtained through
the new sysfs read-only attribute /sys/firmware/dump/dump_area_size.

Suggested-by: Heiko Carstens <hca@linux.ibm.com>
Suggested-by: Peter Oberparleiter <oberpar@linux.ibm.com>
Signed-off-by: Alexander Egorenkov <egorenar@linux.ibm.com>
Reviewed-by: Peter Oberparleiter <oberpar@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>