net: dsa: mv88e6xxx: Add EEE support

EEE configuration is similar for the various MV88E6xxx chips.
Add generic support for it.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>