s390: update defconfig

Enable the contiguous memory allocator but set the default size to
zero. If somebody wants to use the cma allocator the "cma=" kernel
parameter has to be used.

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