FROMLIST: iommu: Introduce map_sg() as an IOMMU op for IOMMU drivers

Add support for IOMMU drivers to have their own map_sg() callbacks.
This completes the path for having iommu_map_sg() invoke an IOMMU
driver's map_sg() callback, which can then invoke the io-pgtable
map_sg() callback with the entire scatter-gather list, so that it
can be processed entirely in the io-pgtable layer.

For IOMMU drivers that do not provide a callback, the default
implementation of iterating through the scatter-gather list, while
calling iommu_map() will be used.

Bug: 176779203
Link: https://lore.kernel.org/linux-iommu/1610376862-927-1-git-send-email-isaacm@codeaurora.org/T/#t


Change-Id: I3d5a8a9e8648649d8dcdda3fa1df41d72f87a528
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>
Tested-by: Sai Prakash Ranjan <saiprakash.ranjan@codeaurora.org>