iommu/arm-smmu: Mark qcom_smmu_client_of_match as possibly unused

When CONFIG_OF=n of_match_device() gets pre-processed out of existence
leaving qcom-smmu_client_of_match unused. Mark it as possibly unused to
keep the compiler from warning in that case.

Fixes: 0e764a01

 ("iommu/arm-smmu: Allow client devices to select direct mapping")
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
Acked-by: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20200604203905.31964-1-jcrouse@codeaurora.org
Signed-off-by: Joerg Roedel <jroedel@suse.de>