phy: SMSC device Energy Detect power-down mode

SMSC Ethernet Transceivers (LAN88710, LAN8710, LAN8720, LAN8187,
LAN8700, LAN83C185) provide a mechanism to conserve power when
the device is not connected to an active link partner
(Energy Detect Mode).
So this patch enables the Energy Detect power-down mode
for these Transceivers.

Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>