s390/configs: provide compat topic configuration target

CONFIG_COMPAT is disabled by default, however compat code still needs to
be compile tested. Add a compat topic configuration target which allows to
enable it easily.
Acked-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

arch/s390/configs/compat.config

+# Help: Enable compat support
+CONFIG_COMPAT=y
+CONFIG_COMPAT_32BIT_TIME=y