OF: Simplify of_iommu_configure()

We no longer have a notion of partially-initialised fwspecs existing,
and we also no longer need to use an iommu_ops pointer to return status
to of_dma_configure(). Clean up the remains of those, which lends itself
to clarifying the logic around the dma_range_map allocation as well.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>