smsc95xx: support PHY wakeup source

This patch enables LAN9500 family devices to wake from suspend
on either link up or link down events

It also adds _nopm versions of mdio access functions, so we can
safely call them from suspend and resume functions

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