add frontend implementation for the IOMMU API

This API can be used by KVM for accessing different types of IOMMUs to
do device passthrough to guests. Beside that this API can also be used
by device drivers to map non-linear host memory into dma-linear
addresses to prevent sgather-gather DMA. UIO may be another user for
this API.

Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
Acked-by: Greg Kroah-Hartman <gregkh@suse.de>