cpsw: ethtool: add support for getting/setting EEE registers

Add the ability to query and set Energy Efficient Ethernet parameters
via ethtool for applicable devices.

This patch doesn't activate full EEE support in cpsw driver, but it
enables reading and writing EEE advertising settings. This way one
can disable advertising EEE for certain speeds.

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Acked-by: Rami Rosen <roszenrami@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>