b44: use fixed PHY device if we do not find any

The ADM6996L switch and some Broadcom switches with two MII interfaces
like the BCM5325F connected to two MACs on the SoC, used on some
routers do not return a valid value when reading the PHY id register
and Linux thinks there is no PHY at all, but that is wrong.
This patch registers a fixed phy in the arch code and then searches it
when there is no other phy in the Ethernet driver code.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: David S. Miller <davem@davemloft.net>