net: bcmgenet: Pass "main" clock down to the MDIO driver

GENET has historically had to create a MDIO platform device for its
controller and pass some auxiliary data to it, like a MDIO completion
callback. Now we also pass the "main" clock to allow for the MDIO bus
controller to manage that clock adequately around I/O accesses.

Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: Florian Fainelli <florian.fainelli@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>