[POWERPC] mpc8349: Add linux,network-index to ethernet nodes in device tree

cuImage needs to know the logical index of the ethernet devices in order
to assign mac addresses.  This adds the needed properties.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
CC: Scott Wood <scottwood@freescale.com>
CC: Kumar Gala <galak@kernel.crashing.org>
CC: Timur Tabi <timur@freescale.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>