dt-bindings: arm-smmu: Document SM8[45]50 GPU SMMU

SM8450 and SM8550 both use a Qualcomm-modified MMU500 for their GPU.
In both cases, it requires a set of clocks to be enabled. Describe that.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Link: https://lore.kernel.org/r/20231127-topic-a7xx_dt-v2-1-2a437588e563@linaro.org


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