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>

Documentation/devicetree/bindings/net/wireless/qcom,ath10k.txt

@@ -71,6 +71,10 @@ Optional properties:
 	Value type: <phandle>
 	Definition: reference to the reserved-memory for the msa region
 		    used by the wifi firmware running in Q6.
+- iommus:
+	Usage: optional
+	Value type: <prop-encoded-array>
+	Definition: A list of phandle and IOMMU specifier pairs.
 
 Example (to supply the calibration data alone):
 
@@ -168,4 +172,5 @@ wifi@18000000 {
 		vdd-1.3-rfa-supply = <&vreg_l17a_1p3>;
 		vdd-3.3-ch0-supply = <&vreg_l25a_3p3>;
 		memory-region = <&wifi_msa_mem>;
+		iommus = <&apps_smmu 0x0040 0x1>;
 };