iommu/ipmmu-vmsa: Clean up struct ipmmu_vmsa_iommu_priv

Now that the IPMMU instance pointer is the only thing remaining in the
private data structure, we no longer need the extra level of indirection
and can simply stash that directlty in the fwspec.

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