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

Add 'optional' property - 'iommus' to the
device-tree binding documentation for qcom-bam DMA IP.

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

Cc: Thara Gopinath <thara.gopinath@linaro.org>
Cc: Bjorn Andersson <bjorn.andersson@linaro.org>
Cc: Rob Herring <robh+dt@kernel.org>
Signed-off-by: Bhupesh Sharma <bhupesh.sharma@linaro.org>