s390/setup: do not complain about parameters handled in decompressor

Currently there are several kernel command line parameters which are
only parsed and handled in decompressor and not known to the kernel.
This leads to the following error message during kernel boot:

Unknown kernel command line parameters "mem=3G nokaslr", will be passed
to user space.

To avoid confusion, register those parameters with an empty stub so that
kernel does not complain about them.

Reported-by: Gerald Schaefer <gerald.schaefer@linux.ibm.com>
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>