Allow mappings from PCI address space to 64b I/O memory
Ultimately motivated by the Pi 4, which is the first RISC OS target platform which requires its PCI windows to be above 4GB.
However, we've no pressing need to have a PCI address space larger than 4GB; the biggest consumer of address space that springs to mind would be a graphics card and even a monster 4 head * 4 bank * 4kx4k * 16M colour image would only take up 1GB, so we're not likely to run out of PCI space on RISC OS platforms for a number of years.