arm64: dts: qcom: qdu1000: add missing qcom,smmu-500 fallback

Since commit 6c84bbd1 ("dt-bindings: arm-smmu: Add generic
qcom,smmu-500 bindings") the SMMU is supposed to use qcom,smmu-500
compatible fallback:

  ['qcom,qdu1000-smmu-500', 'arm,mmu-500'] is too short
  ['qcom,qdu1000-smmu-500', 'arm,mmu-500'] is too long

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20230416123730.300863-3-krzysztof.kozlowski@linaro.org