lan743x: Added fixed link and RGMII support

Microchip lan7431 is frequently connected to a phy. However, it
can also be directly connected to a MII remote peer without
any phy in between. For supporting such a phyless hardware setup
in Linux we utilized phylib, which supports a fixed-link
configuration via the device tree. And we added support for
defining the connection type R/GMII in the device tree.

New behavior:
-------------
. The automatic speed and duplex detection of the lan743x silicon
  between mac and phy is disabled. Instead phylib is used like in
  other typical Linux drivers. The usage of phylib allows to
  specify fixed-link parameters in the device tree.

. The device tree entry phy-connection-type is supported now with
  the modes RGMII or (G)MII (default).

Development state:
------------------
. Tested with fixed-phy configurations. Not yet tested in normal
  configurations with phy. Microchip kindly offered testing
  as soon as the Corona measures allow this.

. All review findings of Andrew Lunn are included

Example:
--------
&pcie {
	status = "okay";

	host@0 {
		reg = <0 0 0 0 0>;

		#address-cells = <3>;
		#size-cells = <2>;

		ethernet@0 {
			compatible = "weyland-yutani,noscom1", "microchip,lan743x";
			status = "okay";
			reg = <0 0 0 0 0>;
			phy-connection-type = "rgmii";

			fixed-link {
				speed = <100>;
				full-duplex;
			};
		};
	};
};

Signed-off-by: Roelof Berg <rberg@berg-solutions.de>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>