dt-bindings: arm-smmu: Add binding doc for Qcom smmu-500

Qcom's implementation of arm,mmu-500 works well with current
arm-smmu driver implementation. Adding a soc specific compatible
along with arm,mmu-500 makes the bindings future safe.

Signed-off-by: Vivek Gautam <vivek.gautam@codeaurora.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: Will Deacon <will.deacon@arm.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>