dt-bindings: arm-smmu: Add compatible for SC7280 SoC

Add the SoC specific compatible for SC7280 implementing
arm,mmu-500.

Signed-off-by: Sai Prakash Ranjan <saiprakash.ranjan@codeaurora.org>
Signed-off-by: Rajendra Nayak <rnayak@codeaurora.org>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/1615788144-29508-1-git-send-email-rnayak@codeaurora.org
Signed-off-by: Will Deacon <will@kernel.org>