iommu: of: enforce const-ness of struct iommu_ops

As a set of driver-provided callbacks and static data, there is no
compelling reason for struct iommu_ops to be mutable in core code, so
enforce const-ness throughout.

Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>