s390/ipl: Introduce macros for (re)ipl sysfs attribute 'scp_data'

This is a refactoring change to reduce code duplication and improve code
reuse.

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>