[S390] initrd: change default load address

Change default load address of the initrd in case of IPL from reader.
The new load address is directly behind the kernel image.
This way we can see immediatly if there are any problems with the code
which tries to rescue the initrd in case the bootmem bitmap would
overlap with the initrd.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>