dt-bindings: arm: coresight-tmc: Add 'iommu' property

The Coresight TMC component may be behind an IOMMU which is the case for
the Arm Juno SoC and some Qualcomm SoCs. Add 'iommus' property to the
binding.

Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220721212718.1980905-2-robh@kernel.org
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>