MAINTAINERS: Add myself to S390 ZFCP DRIVER as a co-maintainer

I have been working with Steffen on zFCP for quite a while now and we
decided adding me as a co-maintainer might be a good thing.

Acked-by: Steffen Maier <maier@linux.vnet.ibm.com>
Signed-off-by: Benjamin Block <bblock@linux.vnet.ibm.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>