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>

Documentation/devicetree/bindings/net/phy.txt

@@ -21,10 +21,18 @@ Optional Properties:
   elements.
 - max-speed: Maximum PHY supported speed (10, 100, 1000...)
 
+  If the phy's identifier is known then the list may contain an entry
+  of the form: "ethernet-phy-idAAAA.BBBB" where
+     AAAA - The value of the 16 bit Phy Identifier 1 register as
+            4 hex digits. This is the chip vendor OUI bits 3:18
+     BBBB - The value of the 16 bit Phy Identifier 2 register as
+            4 hex digits. This is the chip vendor OUI bits 19:24,
+            followed by 10 bits of a vendor specific ID.
+
 Example:
 
 ethernet-phy@0 {
-	compatible = "ethernet-phy-ieee802.3-c22";
+	compatible = "ethernet-phy-id0141.0e90", "ethernet-phy-ieee802.3-c22";
 	interrupt-parent = <40000>;
 	interrupts = <35 1>;
 	reg = <0>;