Disable DETECT_SOFTLOCKUP for s390

From: Heiko Carstens <heiko.carstens@de.ibm.com>

We got several false bug reports because of enabled
CONFIG_DETECT_SOFTLOCKUP.  Disable soft lockup detection on s390, since it
doesn't work on a virtualized architecture.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Adrian Bunk <bunk@kernel.org>