ppce500_pci: remove sysbus_init_mmio_cb2 usage

Expose only one container MemoryRegion to sysbus.
(Peter Maydell's idea)

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Benoît Canet <benoit.canet@gmail.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
