of_mdio: Allow the DT to specify the phy ID and avoid autoprobing

This makes the generic of_mdiobus_register parse the DT compatible string for
the pattern ethernet-phy-idAAAA.BBBB. If present it should be a value that
matches the phy-id register normally readable through MDIO.

When the ID is given the phy autoprobing is defeated and the phy is
created directly.

This is necessary to support phy's that cannot be autoprobed when
of_mdiobus_register is called. Specifically, my case has the phy in reset at
of_mdiobus_register, the reset is only released once the ethernet driver
starts, before it attaches to the phy.

Tested on ARM Kirkwood with phy id 0x01410e90 (Marvell 88E1318)

Signed-off-by: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>