iommu/amd: Add support for contiguous dma allocator

Add code to allocate memory from the contiguous memory
allocator to support coherent allocations larger than 8MB.

Tested-by: Suravee Suthikulpanit <Suravee.Suthikulpanit@amd.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>