of: phy: Only register a phy device for phys

We will soon support devices other than phys on the mdio bus. Look at
a child's compatibility string to determine if it is a phy, before
registering a phy device.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>