net: dsa: b53: Wire-up EEE

Add support for enabling and disabling EEE, as well as re-negotiating it in
.adjust_link() and in .port_enable().

Reviewed-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>