iommu/ipmmu-vmsa: Unify ipmmu_ops

The remaining difference between the ARM-specific and iommu-dma ops is
in the {add,remove}_device implementations, but even those have some
overlap and duplication. By stubbing out the few arm_iommu_*() calls,
we can get rid of the rest of the inline #ifdeffery to both simplify the
code and improve build coverage.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>