iommu/io-pgtable: Make io_pgtable_ops_to_pgtable() macro common

There is no need to keep a useful accessor for a public structure hidden
away in a private implementation. Move it out alongside the structure
definition so that other implementations may reuse it.

Acked-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>