drivers: ethernet: Add platform driver for MMIO mapping reuse

The Ethernet device model consists of multiple subsystem components, such
as MDIO, PHY, MAC and PTP_CLOCK. These components are mapped into a single
PCIe BAR location with same base address.

This platform driver retrieves the MMIO mapping details and provides a
framework to share it with all the child subsystem components. This
approach avoid the duplicate remapping, ensures efficient re-use of
MMIO mappings across related devices.

Example device tree structure for first ethernet instance:

parent0: parent0 {
        compatible = "intel,eth-plat";
        interrupt-parent = <&intc>;
        vendor-id  = <0x8086>;
        device-id  = <0xXXXX>;

        igc0: igc0 {
                compatible = "intel,igc-mac";

                /*
                 * MAC specific properties.
                 */

                status = "okay";
        };

        mdio0: mdio0 {
                compatible = "intel,igc-mdio";
                #address-cells = <1>;
                #size-cells = <0>;

                ethphy0: ethernet-phy@0 {
                        compatible = "ethernet-phy";
                        /*
                         * PHY specific properties.
                         */
                        reg = <0x0>;
                };
        };
};

This framework is modular and re-usable for other PCIe based Ethernet
devices. It can also be extended to support additional platform specific
information shared across child nodes.

Signed-off-by: Vijayakannan Ayyathurai <vijayakannan.ayyathurai@intel.com>