PPC 85xx: Add qemu-ppce500 machine

For KVM we have a special PV machine type called "ppce500". This machine
is inspired by the MPC8544DS board, but implements a lot less features
than that one.

It also provides more PCI slots and is supposed to be enumerated by
device tree only.

This patch adds support for the generic ppce500 machine and tries to
rely solely on device tree for device enumeration.

Signed-off-by: Alexander Graf <agraf@suse.de>
Acked-by: Scott Wood <scottwood@freescale.com>
Reviewed-by: York Sun <yorksun@freescale.com>