Currently we assume there is only one memory and one prefetch memory
window, and ignore the latter. However, the SiFive FU740 has two normal
memory windows.
As part of this, the viewports are rearranged. Previously the viewports
were memory, config then optionally I/O. Both to simplify the config
index calculation and to ensure it can always be mapped even if we have
too many memory windows for the number of viewports, config is moved to
being the first viewport.
This generalisation now also naturally supports mapping prefetch memory
windows.