iommu: Allow the dma-iommu api to use bounce buffers

Allow the dma-iommu api to use bounce buffers for untrusted devices.
This is a copy of the intel bounce buffer code.

Co-developed-by: Lu Baolu <baolu.lu@linux.intel.com>
Signed-off-by: Tom Murphy <murphyt7@tcd.ie>
Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Tested-by: Logan Gunthorpe <logang@deltatee.com>
Link: https://lore.kernel.org/r/20201124082057.2614359-4-baolu.lu@linux.intel.com
Signed-off-by: Will Deacon <will@kernel.org>