dt: bindings: ath10k: add bindings for wifi iommu node

WCN3990 wifi module can optionally make use of the IOMMU.
Add binding documentation for phandle to the IOMMU and
the stream id of wifi iommu block.

Signed-off-by: Govind Singh <govinds@codeaurora.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Brian Norris <briannorris@chromium.org>
Tested-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>