net: mdio: Add BCM6368 MDIO mux bus controller

This controller is present on BCM6318, BCM6328, BCM6362, BCM6368 and BCM63268
SoCs.

Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>