iommu/s390: allow larger region tables

iommu/s390: allow larger region tables

Extend the aperture calculation to consider sizes beyond the maximum
size of a region third table.  Attempt to always use the smallest
table size possible to avoid unnecessary extra steps during translation.
Update reserved region calculations to use the appropriate table size.

Signed-off-by: Matthew Rosato <mjrosato@linux.ibm.com>
Reviewed-by: Niklas Schnelle <schnelle@linux.ibm.com>
Tested-by: Niklas Schnelle <schnelle@linux.ibm.com>
Link: https://lore.kernel.org/r/20250411202433.181683-6-mjrosato@linux.ibm.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>