phylib: mdio: Add sysfs attribute for PHY identifiers.

This adds a phy_id sysfs attribute to MDIO devices, containing the
32-bit PHY identifier reported by the device.  This attribute can
be useful when debugging problems related to phy drivers.  Other
enumerable buses already have similar attributes.

Signed-off-by: Nick Bowler <nbowler@elliptictech.com>
Signed-off-by: David S. Miller <davem@davemloft.net>