iommu: Remove iommu_debugfs_top

iommu_debugfs_top was introduced in earlier kernel versions when
IOMMU_DEBUGFS was not present. Now that IOMMU_DEBUGFS is present,
and exposes iommu_debugfs_dir, which is logically equivalent to
iommu_debugfs_top, there is no point in keeping it around, so
remove it.

Change-Id: I7032e370ccfd32f9d2b9046fecd98b4c6388450f
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>