dt-bindings: qcom-qce: Add 'iommus' to optional properties

Add the missing optional property - 'iommus' to the
device-tree binding documentation for qcom-qce crypto IP.

This property describes the phandle(s) to apps_smmu node with sid mask.

Cc: Bjorn Andersson <andersson@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Tested-by: Jordan Crouse <jorcrous@amazon.com>
Signed-off-by: Bhupesh Sharma <bhupesh.sharma@linaro.org>
Signed-off-by: Vladimir Zapolskiy <vladimir.zapolskiy@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>