iommu/tegra: gart: Provide default ->map_sg() callback

Commit 315786eb ("iommu: Add iommu_map_sg() function") adds a new
->map_sg() callback and provides a default implementation that drivers
can use until they implement a hardware-specific variant. Unfortunately
the Tegra GART driver was not updated as part of that commit, so that
iommu_map_sg() calls on a domain provided by the GART cause an oops.

Fixes: 315786eb ("iommu: Add iommu_map_sg() function")
Cc: Hiroshi Doyu <hdoyu@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>