net: phy: mdio-bcm-unimac: create unique bus names

In preparation for having multiple GENET instances in a system (up to
3), make sure that we do include the bus instance number in the name of
the MDIO bus such that we change it from "unimac-mdio" to
"unimac-mdio-0" for instance.

So far, the only user of this driver is using Device Tree, which uses a
lookup/parenting based technique to map PHY devices to their respective
MDIO bus controllers, hence causing no additional changes.

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