dt: Document a compatible entry for MDIO ethernet Phys

This describes a compatible entry of the form:
  ethernet-phy-idAAAA,BBBB
Which is modelled after the PCI structured compatible entry
(pciVVVV,DDDD.SSSS.ssss.RR)

If present the OF core will be able to use this information to
directly create the correct phy without auto probing the bus.

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