To support "hybrid" DMA ops in a subsequent patch, we will need both
a direct DMA offset and an iommu pointer. Those are currently exclusive
(a union), so change them to be separate fields.

While there, also type iommu_table_base properly and make exist only
on CONFIG_PPC64 since it's not referenced on 32-bit at all.
Signed-off-by: NBenjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: NMichael Ellerman <mpe@ellerman.id.au>