iommu: iommu-debug: select IOMMU_DEBUGFS for iommu tests

Select the IOMMU_DEBUGFS config option when IOMMU_DEBUG is enabled so
that the kernel iommu tests are created under /sys/kernel/debug/iommu.

Change-Id: Ib4035860fee39824a66f16d84423e568c061a158
Signed-off-by: Charan Teja Reddy <charante@codeaurora.org>