iommu/arm-smmu: set a more appropriate DMA mask

Since we use dma_map_page() as an architecture-independent means of
making page table updates visible to non-coherent SMMUs, we need to
have a suitable DMA mask set to discourage the DMA mapping layer from
creating bounce buffers and flushing those instead, if said page tables
happen to lie outside the default 32-bit mask.

Tested-by: Mitchel Humpherys <mitchelh@codeaurora.org>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
[will: added error checking]
Signed-off-by: Will Deacon <will.deacon@arm.com>