dt-bindings: Document the DT bindings for lan78xx

The Microchip LAN78XX family of devices are Ethernet controllers with
a USB interface. Despite being discoverable devices it can be useful to
be able to configure them from Device Tree, particularly in low-cost
applications without an EEPROM or programmed OTP.

Document the supported properties in a bindings file.
Signed-off-by: NPhil Elwell <phil@raspberrypi.org>
Reviewed-by: NAndrew Lunn <andrew@lunn.ch>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/microchip,lan78xx.txt

+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".
+
+Optional properties:
+- local-mac-address:   see ethernet.txt
+- mac-address:         see 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
+						>;
+					};
+				};
+			};
+		};
+	};
+};

MAINTAINERS

@@ -14571,6 +14571,7 @@ M:	Woojung Huh <woojung.huh@microchip.com>
 M:	Microchip Linux Driver Support <UNGLinuxDriver@microchip.com>
 L:	netdev@vger.kernel.org
 S:	Maintained
+F:	Documentation/devicetree/bindings/net/microchip,lan78xx.txt
 F:	drivers/net/usb/lan78xx.*
 F:	include/dt-bindings/net/microchip-lan78xx.h