iommu/io-pgtable-arm: Simplify level indexing

The nature of the LPAE format means that data->pg_shift is always
redundant with data->bits_per_level, since they represent the size of a
page and the number of PTEs per page respectively, and the size of a PTE
is constant. Thus it works out more efficient to only store the latter,
and derive the former via a trivial addition where necessary.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
[will: Reworked granule check in iopte_to_paddr()]
Signed-off-by: Will Deacon <will@kernel.org>