Instead of just creating a simple PHY device for every child of MDIO
bus node, add code to check if any of them have "compatible" property
set, as well as code to create a proper platform device for such
cases.

This change is useful for when MDIO bus and some of Ethernet ports are
connected to a switch or some other MDIO device that doesn't behave
like a generic PHY and can't be probed via its PHY ID.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>