iommu/s390: Add support for iommu_device handling

Add support for the iommu_device_register interface to make
the s390 hardware iommus visible to the iommu core and in
sysfs.

Acked-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>