dt-bindings: arm-smmu: update binding for qcom sc7180 SoC

Add the soc specific compatible for sc7180 smmu-500

Signed-off-by: Rajendra Nayak <rnayak@codeaurora.org>
Cc: Joerg Roedel <joro@8bytes.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Rob Herring <robh@kernel.org>