Documentation: devicetree: add old and deprecated 'fixed-link'

Update the fixed-link Device Tree binding documentation to contain
information about the old and deprecated 5-digit 'fixed-link' property.
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/fixed-link.txt

@@ -18,6 +18,18 @@ properties:
 * 'asym-pause' (boolean, optional), to indicate that asym_pause should
   be enabled.
 
+Old, deprecated 'fixed-link' binding:
+
+* A 'fixed-link' property in the Ethernet MAC node, with 5 cells, of the
+  form <a b c d e> with the following accepted values:
+  - a: emulated PHY ID, choose any but but unique to the all specified
+    fixed-links, from 0 to 31
+  - b: duplex configuration: 0 for half duplex, 1 for full duplex
+  - c: link speed in Mbits/sec, accepted values are: 10, 100 and 1000
+  - d: pause configuration: 0 for no pause, 1 for pause
+  - e: asymmetric pause configuration: 0 for no asymmetric pause, 1 for
+    asymmetric pause
+
 Example:
 
 ethernet@0 {