net: phy: c45: add support for MDIO_AN_EEE_ADV2

Add support for handling the EEE advertisement 2 register.
For now only 2500baseT and 5000baseT modes are supported.

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