We are using the same pfn for every pte we create while constructing the
pmd. Fix this by actually updating the pfn on each iteration of the pmd
construction loop.

It's not clear if we can actually hit this bug right now since iommu_map
splits up the calls to .map based on the page size, so we only ever seem to
iterate this loop once. However, things might change in the future that
might cause us to hit this.
Signed-off-by: NMitchel Humpherys <mitchelh@codeaurora.org>
Signed-off-by: NWill Deacon <will.deacon@arm.com>