net: phy: realtek: add NBase-T PHY auto-detection

Realtek provided information on how the new NIC-integrated PHY's
expose whether they support 2.5G/5G/10G. This allows to automatically
differentiate 1Gbps and 2.5Gbps PHY's, and therefore allows to
remove the fake PHY ID mechanism for RTL8125.
So far RTL8125 supports 2.5Gbps only, but register layout for faster
modes has been defined already, so let's use this information to be
future-proof.

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