net: stmmac: ipq806x: document device tree bindings

Add the device tree bindings documentation for the QCA IPQ806x
variant of the Synopsys DesignWare MAC.

Signed-off-by: Mathieu Olivari <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

Documentation/devicetree/bindings/net/ipq806x-dwmac.txt

···
+* IPQ806x DWMAC Ethernet controller
+
+The device inherits all the properties of the dwmac/stmmac devices
+described in the file net/stmmac.txt with the following changes.
+
+Required properties:
+
+- compatible: should be "qcom,ipq806x-gmac" along with "snps,dwmac"
+	      and any applicable more detailed version number
+	      described in net/stmmac.txt
+
+- qcom,nss-common: should contain a phandle to a syscon device mapping the
+		   nss-common registers.
+
+- qcom,qsgmii-csr: should contain a phandle to a syscon device mapping the
+		   qsgmii-csr registers.
+
+Example:
+
+	gmac: ethernet@37000000 {
+		device_type = "network";
+		compatible = "qcom,ipq806x-gmac";
+		reg = <0x37000000 0x200000>;
+		interrupts = <GIC_SPI 220 IRQ_TYPE_LEVEL_HIGH>;
+		interrupt-names = "macirq";
+
+		qcom,nss-common = <&nss_common>;
+		qcom,qsgmii-csr = <&qsgmii_csr>;
+
+		clocks = <&gcc GMAC_CORE1_CLK>;
+		clock-names = "stmmaceth";
+
+		resets = <&gcc GMAC_CORE1_RESET>;
+		reset-names = "stmmaceth";
+	};