[faraday/ftgmac100] Add support for PHY of-node passing

[ya-mouse]

This patch based on ability of ethernet/cadence to configure PHY with corresponding driver defined in DTS or fallback to the default behaviour otherwise.

An example DTS snippet:

mac0: ethernet@1e660000 {
    no-hw-checksum;

    phy-handle = <&phy0>;
    phy-mode = "rgmii";

    phy0: phy@0 {
        compatible = "ethernet-phy-id001c.c915",
                     "ethernet-phy-ieee802.3-c22";
        reg = <0>;
    };
};

Signed-off-by: Anton D. Kachalov [email protected]