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

Convert the Arm SMMU binding to DT schema.

The existing binding doc doesn't cover the number of variations of
compatible properties found in .dts files. "qcom,msm8998-smmu-v2" was
also missing, so add it.

SoCFPGA Stratix10 has a single clock defined which doesn't match the
binding. This issue remains.

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