EMAC driver: add power down mode

This patch puts phy in power-down mode when interface is down.
Also we should think about energy detect power-down mode, that will
decrease power consumption when no link.

Signed-off-by: Vitja Makarov <vitja.makarov@gmail.com>
Signed-off-by: Bryan Wu <bryan.wu@analog.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>
Signed-off-by: David S. Miller <davem@davemloft.net>