hw/versatile_pci: Expose multiple sysbus mmio regions

Clean up versatile_pci to expose the various PCI mmio regions
properly as separate mmio regions rather than as a single mmio
which uses callbacks to map and unmap everything.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Avi Kivity <avi@redhat.com>
