net/phy/mdio-i2c: Move header file to include/linux/mdio

In preparation for moving all MDIO drivers into drivers/net/mdio, move
the mdio-i2c header file into include/linux/mdio so it can be used by
both the MDIO driver and the SFP code which instantiates I2C MDIO
busses.

v2:
Add include/linux/mdio

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