net: phy: smsc: add support for edpd tunable

This adds support for the EDPD PHY tunable.
Per default EDPD is disabled in interrupt mode, the tunable can be used
to override this, e.g. if the link partner doesn't use EDPD.
The interval to check for energy can be chosen between 1000ms and
2000ms. Note that this value consists of the 1000ms phylib interval
for state machine runs plus the time to wait for energy being detected.

v2:
- consider that phylib core holds phydev->lock when calling the
  phy tunable hooks

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>