intel-iommu: Introduce domain_sg_mapping() to speed up intel_map_sg()

Instead of calling domain_pfn_mapping() repeatedly with single or
small numbers of pages, just pass the sglist in. It can optimise the
number of cache flushes like domain_pfn_mapping() does, and gives a huge
speedup for large scatterlists.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>