phy: Move PHY PM operations into phy_device

The MDIO PM operations are really PHY device PM operations. So move
them into phy_device. This will be needed when we support devices on
the mdio bus which are not PHYs.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>