s390/mem_detect: use SCLP info for continuous memory detection

When neither SCLP storage info, nor z/VM diag260 "storage configuration"
are available assume a continuous online memory of size specified by
SCLP info.

Reviewed-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Reviewed-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>