dt-bindings: arm-smmu: Allow 3 power domains on SM6375 MMU500

The SMMU on SM6375 requires 3 power domains to be active. Add an
appropriate description of that.

Signed-off-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20221115152727.9736-2-konrad.dybcio@linaro.org


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