iommu: Kconfig: fix unmet direct dependencies warning for IOMMU_DEBUGFS

When IOMMU_DEBUG is enabled, IOMMU_DEBUGFS will be selected and
this will result in the below warning if DEBUG_FS isn't set as
IOMMU_DEBUGFS depends on DEBUG_FS.

WARNING: unmet direct dependencies detected for IOMMU_DEBUGFS
  Depends on [n]: IOMMU_SUPPORT [=y] && DEBUG_FS [=n]
    Selected by [y]:
      - IOMMU_DEBUG [=y] && IOMMU_SUPPORT [=y]

Fix this by adding 'depends on DEBUG_FS' in IOMMU_DEBUG.

Fixes: 3afbe556 ("iommu: iommu-debug: select IOMMU_DEBUGFS for iommu tests")
Change-Id: I46ae3c5505242b40f9fb9e2956e600a043d48cc2
Signed-off-by: Vijayanand Jitta <vjitta@codeaurora.org>