r8169: add RTL8125 PHY initialization

This patch adds PHY initialization magic copied from the r8125 vendor
driver. In addition it supports loading the firmware for chip version
RTL_GIGA_MAC_VER_61.

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