dt-bindings: net: add EEE capability constants

Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Tested-by: Yegor Yefremov <yegorslists@googlemail.com>
Tested-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: David S. Miller <davem@davemloft.net>