dt-bindings: qcom: geni-se: document iommus

Document the iommus property for QCOM Geni SE.

Signed-off-by: Caleb Connolly <caleb@connolly.tech>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210408161953.26298-4-caleb@connolly.tech


Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>