r8169: add support for RTL8125

This adds support for 2.5Gbps chip RTL8125, it's partially based on the
r8125 vendor driver. Tested with a Delock 89531 PCIe card against a
Netgear GS110MX Multi-Gig switch. Firmware isn't strictly needed,
but on some systems there may be compatibility issues w/o firmware.
Firmware has been submitted to linux-firmware.

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