s390/ipl: Add missing SCSI loadparm attributes to /sys/firmware

Currently the loadparm is only supported for CCW IPL. But also for SCSI
IPL it can be specified either on the HMC load panel respectively
z/VM console or via diagnose 308.

So fix this for SCSI and add the required sysfs attributes for reading the
IPL loadparm and for setting the loadparm for re-IPL.

With this patch the following two sysfs attributes are introduced:

 - /sys/firmware/ipl/loadparm (for system that have been IPLed from SCSI)
 - /sys/firmware/reipl/fcp/loadparm

Because the loadparm is now available for SCSI and CCW it is moved
now from "struct ipl_block_ccw" to the generic "struct ipl_list_hdr".

Reviewed-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>