net: of_mdio: scan mdiobus for PHYs without reg property

Using DT for mdiobus and ethernet-phy requires to know the PHY address, which
is hard to guess if you don't know it. This patch extends of_mdiobus_register
to scan mdiobus for PHYs if reg property of the corresponding node is not set.
This also allows to have phy nodes in SoC DT files where the reg property can
be overwritten in the board file later. To encourage people to finally set the
actual phy address, the mdiobus scan is noisier than required.

Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>