versatile_pci: Expose PCI memory space to system

The VersatilePB's PCI controller exposes the PCI memory space to the
system via three regions controlled by the mapping control registers.
Implement this so that guests can actually use MMIO-BAR PCI cards.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Acked-by: Paul Brook <paul@codesourcery.com>
