net: phy: add struct device_type representation of a PHY

A PHY is a type of MDIO device, so let's model it as struct device_type
and place PM ops, attribute groups and release callback on device type
level. For this the attribute definitions have to be moved.
This change allows us to get rid of the PM ops on a bus level in a second
step.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>