iommu: Add support for delayed unmapping of ion/dma_buf buffer

Add new APIs to allow clients to map and unmap dma_buffers created by
ION. The call to the unmap API will not actually do the unmapping from the
IOMMU. The unmapping will occur when the ION/dma_buf buffer is actually
freed. This behavior can be disabled with DMA_ATTR_NO_DELAYED_UNMAP

Change-Id: Ic4dbd3b582eb0388020c650cab5fbb1dad67ae81
Signed-off-by: Olav Haugan <ohaugan@codeaurora.org>