s390/hmcdrv: Restrict s390 HMC driver to S390 arch

This driver is only usable on 64-bit s390 machines.  Mark the Kconfig
dependencies to that users on other architectures are not prompted for it.

Fixes: 8f933b10

 ("s390/hmcdrv: HMC drive CD/DVD access")
Signed-off-by: Josh Boyer <jwboyer@fedoraproject.org>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>