ppc_oldworld, ppc_newworld: fix escc BAR related crash

ppc maps the escc mmio region both at a fixed offset (as a sysbus area) and as part of a PCI BAR.
This crashes, since a MemoryRegion may have only one parent.  Use an alias so we have a separate
MemoryRegion for the BAR.

Signed-off-by: Avi Kivity <avi@redhat.com>
