How do you configure the pcie root port on the Stratix 10 to allow the HPS to map an I/O BAR?

Right, I don't seem to be able to follow your steps. Perhaps we have our Platform designer windows layed out totally differently. I made best guesses and have posted a screenshot.

In your 3) above, I do not have anything labeled "System" in the left hand pane. I do have a "System View" in the middle". I also do have a "System Info" on the right pane, and I have left that selected in the screenshot. I'm assuming you intended for me to have my pcie IP selected first.

In your 4) above, I do not have an "Address Spaces" section. I do have an "Address Map" in the middle section. I have most of my pcie components listed there, but there is no "I/O" row to be found.