iommu/io-pgtable: Avoid redundant TLB syncs

In certain unmapping situations it is quite possible to end up issuing
back-to-back TLB synchronisations, which at best is a waste of time and
effort, and at worst causes some hardware to get rather confused. Whilst
the pagetable implementations, or the IOMMU drivers, or both, could keep
track of things to avoid this happening, it seems to make the most sense
to prevent code duplication and add some simple state tracking in the
common interface between the two.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>