ftgmac100: Document device-tree binding

* Faraday Technology FTGMAC100 gigabit ethernet controller

Required properties:

- compatible: "faraday,ftgmac100"

  Must also contain one of these if used as part of an Aspeed AST2400

  or 2500 family SoC as they have some subtle tweaks to the

  implementation:

     - "aspeed,ast2400-mac"

     - "aspeed,ast2500-mac"

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

- interrupts: Should contain ethernet controller interrupt

Optional properties:

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

  absent, "rgmii" is assumed. Supported values are "rgmii*" and "rmii" for

  aspeed parts. Other (unknown) parts will accept any value.

- use-ncsi: Use the NC-SI stack instead of an MDIO PHY. Currently assumes

  rmii (100bT) but kept as a separate property in case NC-SI grows support

  for a gigabit link.

- no-hw-checksum: Used to disable HW checksum support. Here for backward

  compatibility as the driver now should have correct defaults based on

  the SoC.

Example:

	mac0: ethernet@1e660000 {

		compatible = "aspeed,ast2500-mac", "faraday,ftgmac100";

		reg = <0x1e660000 0x180>;

		interrupts = <2>;

		status = "okay";

		use-ncsi;

	};