dwc_eth_qos: Add Synopsys DWC Ethernet QoS bindings

* Synopsys DWC Ethernet QoS IP version 4.10 driver (GMAC)

Required properties:

- compatible: Should be "snps,dwc-qos-ethernet-4.10"

- reg: Address and length of the register set for the device

- clocks: Phandles to the reference clock and the bus clock

- clock-names: Should be "phy_ref_clk" for the reference clock and "apb_pclk"

  for the bus clock.

- interrupt-parent: Should be the phandle for the interrupt controller

  that services interrupts for this device

- interrupts: Should contain the core's combined interrupt signal

- phy-mode: See ethernet.txt file in the same directory

Optional properties:

- dma-coherent: Present if dma operations are coherent

- mac-address: See ethernet.txt in the same directory

- local-mac-address: See ethernet.txt in the same directory

- snps,en-lpi: If present it enables use of the AXI low-power interface

- snps,write-requests: Number of write requests that the AXI port can issue.

  It depends on the SoC configuration.

- snps,read-requests: Number of read requests that the AXI port can issue.

  It depends on the SoC configuration.

- snps,burst-map: Bitmap of allowed AXI burst lengts, with the LSB

  representing 4, then 8 etc.

- snps,txpbl: DMA Programmable burst length for the TX DMA

- snps,rxpbl: DMA Programmable burst length for the RX DMA

- snps,en-tx-lpi-clockgating: Enable gating of the MAC TX clock during

  TX low-power mode.

- phy-handle: See ethernet.txt file in the same directory

- mdio device tree subnode: When the GMAC has a phy connected to its local

    mdio, there must be device tree subnode with the following

    required properties:

    - compatible: Must be "snps,dwc-qos-ethernet-mdio".

    - #address-cells: Must be <1>.

    - #size-cells: Must be <0>.

    For each phy on the mdio bus, there must be a node with the following

    fields:

    - reg: phy id used to communicate to phy.

    - device_type: Must be "ethernet-phy".

    - fixed-mode device tree subnode: see fixed-link.txt in the same directory

Examples:

ethernet2@40010000 {

	clock-names = "phy_ref_clk", "apb_pclk";

	clocks = <&clkc 17>, <&clkc 15>;

	compatible = "snps,dwc-qos-ethernet-4.10";

	interrupt-parent = <&intc>;

	interrupts = <0x0 0x1e 0x4>;

	reg = <0x40010000 0x4000>;

	phy-handle = <&phy2>;

	phy-mode = "gmii";

	snps,en-tx-lpi-clockgating;

	snps,en-lpi;

	snps,write-requests = <2>;

	snps,read-requests = <16>;

	snps,burst-map = <0x7>;

	snps,txpbl = <8>;

	snps,rxpbl = <2>;

	dma-coherent;

	mdio {

		#address-cells = <0x1>;

		#size-cells = <0x0>;

		phy2: phy@1 {

			compatible = "ethernet-phy-ieee802.3-c22";

			device_type = "ethernet-phy";

			reg = <0x1>;

		};

	};

};