r8169: add support for EEE on RTL8125

This adds EEE support for RTL8125 based on the vendor driver.
Supported is EEE for 100Mbps and 1Gbps. Realtek recommended to not yet
enable EEE for 2.5Gbps due to potential compatibility issues. Also
ethtool doesn't support yet controlling EEE for 2.5Gbps and 5Gbps.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>