dt-bindings: iommu: Convert Arm SMMUv3 to DT schema

Convert the Arm SMMv3 binding to the DT schema format.

Cc: Joerg Roedel <joro@8bytes.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: iommu@lists.linux-foundation.org
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Rob Herring <robh@kernel.org>