net: bcmgenet: add EEE support

Allow enabling and disabling EEE using the designated ethtool getters
and setters. GENET allows controlling EEE at the UniMAC, RBUF and TBUF
levels. We also take care of restoring EEE after a suspend/resume cycle
if it was enabled prior to suspending.

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