Revert "defconfig: kona: enable CONFIG_IOMMU_DEBUGFS on kona"

We ensure that the IOMMU_DEBUGFS is selected when IOMMU_DEBUG is
enabled. So revert this.
This reverts commit d543ba29.

Change-Id: I2cf542062ca257c24234cfb47f06bec64411cce9
Signed-off-by: Charan Teja Reddy <charante@codeaurora.org>