net: stmmac: meson: document device tree bindings

* Amlogic Meson 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 "amlogic,meson6-dwmac" along with "snps,dwmac"

	      and any applicable more detailed version number

	      described in net/stmmac.txt

- reg: should contain a register range for the dwmac controller and

       another one for the Amlogic specific configuration

Example:

	ethmac: ethernet@c9410000 {

		compatible = "amlogic,meson6-dwmac", "snps,dwmac";

		reg = <0xc9410000 0x10000

		       0xc1108108 0x4>;

		interrupts = <0 8 1>;

		interrupt-names = "macirq";

		clocks = <&clk81>;

		clock-names = "stmmaceth";

	}