net: stmmac: ipq806x: document device tree bindings

* 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";

	};