s390/ipl: Introduce sysfs attribute 'scp_data' for dump ipl

This is analogous to the reipl's sysfs attribute named equally and enables
tools such as s390-tools' dumpconf to pass additional kernel cmdline
parameters to a stand-alone dumper such as zfcpdump (e.g. to enable
debug output with 'dump_debug' parameter) or ngdump.

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