mdio-uclass.c: support fixed-link subnodes

mdio-uclass.c: support fixed-link subnodes

When trying to port our mpc8309-based board to DM_ETH, on top of
Heiko's patches, I found that nothing in mdio-uclass.c seems to
support the use of a fixed-link subnode of the ethernet DT node. That
is, the ethernet node looks like

		enet0: ethernet@2000 {
			device_type = "network";
			compatible = "ucc_geth";
			...
			fixed-link {
				reg = <0xffffffff>;
				speed = <100>;
				full-duplex;
			};

but the current code expects there to be phy-handle property. Adding
that, i.e.

			phy-handle = <&enet0phy>;
			enet0phy: fixed-link {

just makes the code break a few lines later since a fixed-link node
doesn't have a reg property. Ignoring the dtc complaint and adding a
dummy reg property, we of course hit "can't find MDIO bus for node
ethernet@2000" since indeed, the parent node of the phy node does not
represent an MDIO bus. So that's obviously the wrong path.

Now, in linux, it seems that the fixed link case is treated specially;
in the of_phy_get_and_connect() which roughly corresponds to
dm_eth_connect_phy_handle() we have

    if (of_phy_is_fixed_link(np)) {
        ret = of_phy_register_fixed_link(np);
        ...
    } else {
        phy_np = of_parse_phandle(np, "phy-handle", 0);
	...
    }

    phy = of_phy_connect(dev, phy_np, hndlr, 0, iface);

And U-Boot's phy_connect() does have support for fixed-link
subnodes. Calling phy_connect() directly with NULL bus and a dummy
address does seem to make the ethernet work.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Reviewed-by: Heiko Schocher <hs@denx.de>