arm/tegra: Call bus_set_iommu() after iommu_device_register()

The bus_set_iommu() function will call the add_device()
call-back which needs the iommu to be registered.

Reported-by: Jon Hunter <jonathanh@nvidia.com>
Fixes: 0b480e44

 ('iommu/tegra: Add support for struct iommu_device')
Signed-off-by: Joerg Roedel <jroedel@suse.de>