[POWERPC] Document the linux,network-index property.

To allow more robust association of each network device node with an
index (such as is used by the firmware or an EEPROM to indicate MAC
addresses), a network device's node may specify the index explicitly.
Signed-off-by: NScott Wood <scottwood@freescale.com>
Acked-by: David Gibson david@gibson.dropbear.id.au>
Signed-off-by: NPaul Mackerras <paulus@samba.org>

Documentation/powerpc/booting-without-of.txt

@@ -1182,6 +1182,13 @@ platforms are moved over to use the flattened-device-tree model.
     - phy-handle : The phandle for the PHY connected to this ethernet
       controller.
 
+  Recommended properties:
+
+    - linux,network-index : This is the intended "index" of this
+      network device.  This is used by the bootwrapper to interpret
+      MAC addresses passed by the firmware when no information other
+      than indices is available to associate an address with a device.
+
   Example:
 
 	ethernet@24000 {
@@ -1550,6 +1557,12 @@ platforms are moved over to use the flattened-device-tree model.
    - mac-address : list of bytes representing the ethernet address.
    - phy-handle : The phandle for the PHY connected to this controller.
 
+   Recommended properties:
+   - linux,network-index : This is the intended "index" of this
+     network device.  This is used by the bootwrapper to interpret
+     MAC addresses passed by the firmware when no information other
+     than indices is available to associate an address with a device.
+
    Example:
 	ucc@2000 {
 		device_type = "network";