iommu/tegra241-cmdqv: Fix unused variable warning

While testing some io-pgtable changes, I ran into a compiler warning
from the Tegra CMDQ driver:

  drivers/iommu/arm/arm-smmu-v3/tegra241-cmdqv.c:803:23: warning: unused variable 'cmdqv_debugfs_dir' [-Wunused-variable]
    803 | static struct dentry *cmdqv_debugfs_dir;
        |                       ^~~~~~~~~~~~~~~~~
  1 warning generated.

Guard the variable declaration with CONFIG_IOMMU_DEBUGFS to silence the
warning.

Signed-off-by: Will Deacon <will@kernel.org>