dt-bindings: ath10k: Add wifi-firmware subnode for wifi node

Add a wifi-firmware subnode for the wifi node.
This wifi-firmware subnode is needed for the
targets which do not support TrustZone.

Signed-off-by: Rakesh Pillai <[e-mail]>
Acked-by: Rob Herring <[e-mail]>
Signed-off-by: Kalle Valo <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

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

···
 - qcom,coexist-gpio-pin : gpio pin number  information to support coex
 			  which will be used by wifi firmware.
 
+* Subnodes
+The ath10k wifi node can contain one optional firmware subnode.
+Firmware subnode is needed when the platform does not have TustZone.
+The firmware subnode must have:
+
+- iommus:
+	Usage: required
+	Value type: <prop-encoded-array>
+	Definition: A list of phandle and IOMMU specifier pairs.
+
+
 Example (to supply PCI based wifi block details):
 
 In this example, the node is defined as child node of the PCI controller.
···
 		memory-region = <&wifi_msa_mem>;
 		iommus = <&apps_smmu 0x0040 0x1>;
 		qcom,msa-fixed-perm;
+		wifi-firmware {
+			iommus = <&apps_iommu 0xc22 0x1>;
+		};
 };