s390/ipl: add definitions for the IPL report block

To transport the information required for secure boot a new IPL report
will be created at boot time. It will be written to memory right after
the IPL parameter block. To work with the IPL report a couple of
additional structure definitions are added the the uapi/ipl.h header.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>