iommu/tegra-gart: Add support for struct iommu_device

Add a struct iommu_device to each tegra-gart and register it
with the iommu-core. Also link devices added to the driver
to their respective hardware iommus.

Reviewed-by: Dmitry Osipenko <digetx@gmail.com>
Tested-by: Dmitry Osipenko <digetx@gmail.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>