drivers: ethernet: make drivers phy agnostic

[maass-hamburg]

The ethernet drivers should not use mdio directly to configure and read the phy,
instead, the ethernet phy api should be used.

• drivers: ethernet: xlnx_gem: switch integrated MDIO/PHY support over to Zephyr's framework #87313
• drivers: ethernet: stm32: remove phy specific code #87593
• nxp_kinetis_ethernet (eth_mcux.c)
• nuvoton_numaker_ethernet (eth_numaker.c)
• snps_ethernet_cyclonev (eth_cyclonev.c phy_cyclonev.c)
• silabs_gecko_ethernet (eth_gecko.c phy_gecko.c)
• smsc_lan9220