net: phy: add PHY_EEE_CAP2_FEATURES

As a prerequisite for adding EEE CAP2 register support, complement
PHY_EEE_CAP1_FEATURES with PHY_EEE_CAP2_FEATURES.
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>