[S390] set ARCH_HAS_SG_CHAIN for s390

Set ARCH_HAS_SG_CHAIN for scatter lists on s390. Without this flag the
SCSI code limits the maximum number of segments, so set it to make
proper use of the FCP channel hardware.
Signed-off-by: Christof Schmitt <christof.schmitt@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/include/asm/scatterlist.h

 #include <asm-generic/scatterlist.h>
+
+#define ARCH_HAS_SG_CHAIN