dt-bindings: arm-smmu: Add compatible for SM8550 SoC

Add the SoC specific compatible for SM8550 implementing
arm,mmu-500.

Signed-off-by: Abel Vesa <abel.vesa@linaro.org>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230207-topic-sm8550-upstream-smmu-bindings-v3-1-cb15a7123cfe@linaro.org


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