s390/kexec_file: Simplify parmarea access

Access the parmarea in head.S via a struct instead of individual offsets.

While at it make the fields in the parmarea .quads.

Signed-off-by: Philipp Rudo <prudo@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>