net: phy: Export mdiobus_register_board_info()

We can build modular code that uses mdiobus_register_board_info() which would
lead to linking failure since this symbol is not expoerted.

Fixes: 648ea013 ("net: phy: Allow pre-declaration of MDIO devices")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>