net: stmmac: don't create a MDIO bus if unnecessary

Currently a MDIO bus is created if the devicetree description is either:

    1. Not fixed-link
    2. fixed-link but contains a MDIO bus as well

The "1" case above isn't always accurate. If there's a phy-handle,
it could be referencing a phy on another MDIO controller's bus[1]. In
this case, where the MDIO bus is not described at all, currently
stmmac will make a MDIO bus and scan its address space to discover
phys (of which there are none). This process takes time scanning a bus
that is known to be empty, delaying time to complete probe.

There are also a lot of upstream devicetrees[2] that expect a MDIO bus
to be created, scanned for phys, and the first one found connected
to the MAC. This case can be inferred from the platform description by
not having a phy-handle && not being fixed-link. This hits case "1" in
the current driver's logic, and must be handled in any logic change here
since it is a valid legacy dt-binding.

Let's improve the logic to create a MDIO bus if either:

    - Devicetree contains a MDIO bus
    - !fixed-link && !phy-handle (legacy handling)

This way the case where no MDIO bus should be made is handled, as well
as retaining backwards compatibility with the valid cases.

Below devicetree snippets can be found that explain some of
the cases above more concretely.

Here's[0] a devicetree example where the MAC is both fixed-link and
driving a switch on MDIO (case "2" above). This needs a MDIO bus to
be created:

    &fec1 {
            phy-mode = "rmii";

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

            mdio1: mdio {
                    switch0: switch0@0 {
                            compatible = "marvell,mv88e6190";
                            pinctrl-0 = <&pinctrl_gpio_switch0>;
                    };
            };
    };

Here's[1] an example where there is no MDIO bus or fixed-link for
the ethernet1 MAC, so no MDIO bus should be created since ethernet0
is the MDIO master for ethernet1's phy:

    &ethernet0 {
            phy-mode = "sgmii";
            phy-handle = <&sgmii_phy0>;

            mdio {
                    compatible = "snps,dwmac-mdio";
                    sgmii_phy0: phy@8 {
                            compatible = "ethernet-phy-id0141.0dd4";
                            reg = <0x8>;
                            device_type = "ethernet-phy";
                    };

                    sgmii_phy1: phy@a {
                            compatible = "ethernet-phy-id0141.0dd4";
                            reg = <0xa>;
                            device_type = "ethernet-phy";
                    };
            };
    };

    &ethernet1 {
            phy-mode = "sgmii";
            phy-handle = <&sgmii_phy1>;
    };

Finally there's descriptions like this[2] which don't describe the
MDIO bus but expect it to be created and the whole address space
scanned for a phy since there's no phy-handle or fixed-link described:

    &gmac {
            phy-supply = <&vcc_lan>;
            phy-mode = "rmii";
            snps,reset-gpio = <&gpio3 RK_PB4 GPIO_ACTIVE_HIGH>;
            snps,reset-active-low;
            snps,reset-delays-us = <0 10000 1000000>;
    };

[0] https://elixir.bootlin.com/linux/v6.5-rc5/source/arch/arm/boot/dts/nxp/vf/vf610-zii-ssmb-dtu.dts
[1] https://elixir.bootlin.com/linux/v6.6-rc5/source/arch/arm64/boot/dts/qcom/sa8775p-ride.dts
[2] https://elixir.bootlin.com/linux/v6.6-rc5/source/arch/arm64/boot/dts/rockchip/rk3368-r88.dts#L164



Reviewed-by: Serge Semin <fancer.lancer@gmail.com>
Co-developed-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Signed-off-by: Andrew Halaney <ahalaney@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>