versatile_pci: Expose PCI I/O region on Versatile PB

Comments in the QEMU source code claim that the version of the PCI
controller on the VersatilePB board doesn't support the PCI I/O
region, but this is incorrect; expose that region, map it in the
correct location, and drop the misleading comments.

This change removes the only currently implemented difference
between the realview-pci and versatile-pci models; however there
are other differences in not-yet-implemented functionality, so we
retain the distinction between the two device types.

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