r8169: add support for RTL8126A

This adds support for the RTL8126A found on Asus z790 Maximus Formula.
It was successfully tested w/o the firmware at 1000Mbps. Firmware file
has been provided by Realtek and submitted to linux-firmware.
2.5G and 5G modes are untested.

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