Make iommu_map_sg take a struct iommu_table. It did so before commit
740c3ce6 (iommu sg merging: ppc: make
iommu respect the segment size limits).

This stops the function looking in the archdata.dma_data for the iommu
table because in the future it will be called with a device that has
no table there.

This also has the nice side effect of making iommu_map_sg() match the
other map functions.
Signed-off-by: Mark Nelson <markn@au1.ibm.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>