It is possible to shut down the ethernet link with "ifconfig eth0 down". That way, the PHY should power down and disable further communication.
You might want to disable the automatic link up + dhcp for wired connections, as Yocto adds that policy under /lib/systemd/network/80-wired.network. If you do this, you will need to run the commands "ifconfig eth0 up" and "udhcpc -i eth0" manually.
I just tested that the genphy_suspend function from the ethernet PHY driver in the kernel gets called, when "ifconfig eth0 down" is issued. This way the generic power down over the mdio communication can be done.
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