dt-bindings: iommu: qcom_iommu: Allow 'tbu' clock

dt-bindings: iommu: qcom_iommu: Allow 'tbu' clock

Some IOMMUs on some platforms (there doesn't seem to be a good denominator
for this) require the presence of a third clock, specifically relating
to the instance's Translation Buffer Unit (TBU).

Stephan Gerhold noted [1] that according to Qualcomm Snapdragon 410E
Processor (APQ8016E) Technical Reference Manual, SMMU chapter, section
"8.8.3.1.2 Clock gating", which reads:

For APPS TCU/TBU (TBU to TCU interface is asynchronous)
Software should turn ON clock to APPS TCU
  - During APPS TCU register programming sequence

For GPU TCU/TBU (TBU to TCU interface is synchronous)
Software should turn ON clock to GPU TBU
  - During GPU TLB invalidation sequence <=====================
Software should turn ON clock to GPU TCU
  - During GPU TCU register programming sequence
  - While GPU master clock is Active

The clock should be turned on at least during TLB invalidation on the
GPU SMMU instance. This is corroborated by Commit 5bc1cf1466f6
("iommu/qcom: add optional 'tbu' clock for TLB invalidate").

This is also not to be confused with qcom,sdm845-tbu, which is a
description of a debug interface, absent on the generation of hardware
that this binding describes.

Allow this clock.

[1] https://lore.kernel.org/linux-arm-msm/aPX_cKtial56AgvU@linaro.org/

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Reviewed-by: Bjorn Andersson <andersson@kernel.org>
Signed-off-by: Will Deacon <will@kernel.org>