of: mdio: Support fixed links in of_phy_get_and_connect()

By a simple extension of of_phy_get_and_connect() drivers
that have a fixed link on e.g. RGMII can support also
fixed links, so in addition to:

ethernet-port {
	phy-mode = "rgmii";
	phy-handle = <&foo>;
};

This setup with a fixed-link node and no phy-handle will
now also work just fine:

ethernet-port {
	phy-mode = "rgmii";
	fixed-link {
		speed = <1000>;
		full-duplex;
		pause;
	};
};

This is very helpful for connecting random ethernet ports
to e.g. DSA switches that typically reside on fixed links.

The phy-mode is still there as the fixes link in this case
is still an RGMII link.

Tested on the Cortina Gemini driver with the Vitesse DSA
router chip on a fixed 1Gbit link.

Suggested-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>