[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: Scott Wood <scottwood@freescale.com>
Acked-by: David Gibson david@gibson.dropbear.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>