phy: add set_wol/get_wol functions

This allows ethernet drivers (such as the mv643xx_eth) to support
Wake on LAN on platforms where PHY registers have to be configured
for Wake on LAN (e.g. the Marvell Kirkwood based qnap TS-119P II).

Signed-off-by: Michael Stapelberg <michael@stapelberg.de>
Signed-off-by: David S. Miller <davem@davemloft.net>