Microchip LAN78xx Gigabit Ethernet controller

The LAN78XX devices are usually configured by programming their OTP or with
an external EEPROM, but some platforms (e.g. Raspberry Pi 3 B+) have neither.
The Device Tree properties, if present, override the OTP and EEPROM.

Required properties:
- compatible: Should be one of "usb424,7800", "usb424,7801" or "usb424,7850".

The MAC address will be determined using the optional properties
defined in ethernet.txt.

Optional properties of the embedded PHY:
- microchip,led-modes: a 0..4 element vector, with each element configuring
  the operating mode of an LED. Omitted LEDs are turned off. Allowed values
  are defined in "include/dt-bindings/net/microchip-lan78xx.h".

Example:

/* Based on the configuration for a Raspberry Pi 3 B+ */
&usb {
	usb-port@1 {
		compatible = "usb424,2514";
		reg = <1>;
		#address-cells = <1>;
		#size-cells = <0>;

		usb-port@1 {
			compatible = "usb424,2514";
			reg = <1>;
			#address-cells = <1>;
			#size-cells = <0>;

			ethernet: ethernet@1 {
				compatible = "usb424,7800";
				reg = <1>;
				local-mac-address = [ 00 11 22 33 44 55 ];

				mdio {
					#address-cells = <0x1>;
					#size-cells = <0x0>;
					eth_phy: ethernet-phy@1 {
						reg = <1>;
						microchip,led-modes = <
							LAN78XX_LINK_1000_ACTIVITY
							LAN78XX_LINK_10_100_ACTIVITY
						>;
					};
				};
			};
		};
	};
};