bgmac: register fixed PHY for ARM BCM470X / BCM5301X chipsets

On ARM SoCs with bgmac Ethernet hardware we don't have any normal PHY.
There is always a switch attached but it's not even controlled over MDIO
like in case of MIPS devices.
We need a fixed PHY to be able to send/receive packets from the switch.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>