net: phy: bcm7xxx: enable EEE at the PHY level

The 28nm Gigabit PHY on BCM7xxx chips comes out of reset with absolutely
no EEE capabilities, such that we would actually return that we do not
support EEE when accessing 3.20 (MDIO_PCS_EEE_ABLE) registers.

Poke through the vendor-specific C45 register to enable EEE globally at
the PHY level, and advertise supported EEE modes.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>