iommu/tegra: smmu: Use debugfs_create_dir for directory

The commit c3b1a350 "debugfs: make sure that debugfs_create_file() gets
used only for regulars" doesn't allow to use debugfs_create_file() for
dir. Keep debugfs data in smmu_device instead of directory's i_private.

Signed-off-by: Hiroshi Doyu <hdoyu@nvidia.com>
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>