[POWERPC] Start removing linux,network-index in favour of aliases

This patch alters the bootwrapper for a number of machines (roubhly
all 4xx based cuboot or treeboot platforms) to use aliases instead of
the linux,network-index hack to work out which MAC address to attach
to which ethernet device node.

The now obsolete linux,network-index properties are removed from the
corresponding device trees.  This won't break backwards compatiblity,
because in cases where this fixup code is relevant, the device tree is
part of the kernel image.

The references to linux,network-index are removed from
booting-without-of.txt.  Not only is it now deprecated, but as a hack
applicable only when the device tree blob and fixup code were in the
same image, this property never belonged in booting-without-of.txt
which describes the interface between the kernel and firmware or
bootloaders which produce a device tree.  By the time the device tree
reaches the kernel, all the MAC addresses must be fully filled in.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>