dt-bindings: net: add mt76 wireless device binding

Add documentation describing how device tree can be used to configure
wireless chips supported by the mt76 driver.
Signed-off-by: NFelix Fietkau <nbd@nbd.name>
Signed-off-by: NKalle Valo <kvalo@codeaurora.org>

Documentation/devicetree/bindings/net/wireless/mediatek,mt76.txt

+* MediaTek mt76xx devices
+
+This node provides properties for configuring the MediaTek mt76xx wireless
+device. The node is expected to be specified as a child node of the PCI
+controller to which the wireless chip is connected.
+
+Optional properties:
+
+- mac-address: See ethernet.txt in the parent directory
+- local-mac-address: See ethernet.txt in the parent directory
+- ieee80211-freq-limit: See ieee80211.txt
+- mediatek,mtd-eeprom: Specify a MTD partition + offset containing EEPROM data
+
+Optional nodes:
+- led: Properties for a connected LED
+  Optional properties:
+    - led-sources: See Documentation/devicetree/bindings/leds/common.txt
+
+&pcie {
+	pcie0 {
+		wifi@0,0 {
+			compatible = "mediatek,mt76";
+			reg = <0x0000 0 0 0 0>;
+			ieee80211-freq-limit = <5000000 6000000>;
+			mediatek,mtd-eeprom = <&factory 0x8000>;
+
+			led {
+				led-sources = <2>;
+			};
+		};
+	};
+};