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>