net: of_mdio: factor PHY registration from of_mdiobus_register

Since commit 779d835e ("net: of_mdio: scan mdiobus for PHYs without reg
property") we have two foreach loops which do pretty much the same
thing. Factor the PHY device registration in a function helper:
of_mdiobus_register_phy() which takes care of the details and allows for
future PHY specific extensions.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>