include: mdio: Add driver data helpers

Add set/get drv_data helpers for mdio device.

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