s390/defconfig: allow early device mapper disks

doing make install on an Ubuntu that is installed on an LVM will fail to
boot. Turns out that defconfig misses 2 device mapper related configs
for the Ubuntu initramfs.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: Steffen Maier <maier@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>