phy: Add suspend/resume support to SMSC PHYs

All supported SMSC PHYs implement the standard "power down" bit 11 of
BMCR, so this patch adds support using the generic genphy_{suspend,resume}
functions.

Signed-off-by: Steve Glendinning <steve.glendinning@smsc.com>
Signed-off-by: David S. Miller <davem@davemloft.net>