cpmac: add support for fixed PHY

This patch adds support for fixed PHY connected in MII mode
to cpmac. We allow external and dumb_switch module parameters
to override the PHY detection process since they are always connected
with MDIO bus identifier 0. This lets fixed PHYs to be detected
correctly and be connected to the their corresponding MDIO
bus identifier.

Signed-off-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: David S. Miller <davem@davemloft.net>