net: phy: bcm7xxx: enable auto power down

The 28nm process BCM7xxx internal Gigabit PHYs all support automatic
power down, turn on that feature as part of the configuration
initialization callback.

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