net: phy: mdio-bcm-unimac: Allow specifying platform data

In preparation for having the bcmgenet driver migrate over the
mdio-bcm-unimac driver, add a platform data structure which allows
passing integrating specific details like bus name, wait function to
complete MDIO operations and PHY mask.

We also define what the platform device name contract is by defining
UNIMAC_MDIO_DRV_NAME and moving it to the platform_data header.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>