iommu/io-pgtable-arm: Simplify PGD size handling

We use data->pgd_size directly for the one-off allocation and freeing of
the top-level table, but otherwise it serves for ARM_LPAE_PGD_IDX() to
repeatedly re-calculate the effective number of top-level address bits
it represents. Flip this around so we store the form we most commonly
need, and derive the lesser-used one instead. This cuts a whole bunch of
code out of the map/unmap/iova_to_phys fast-paths.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>