Extend the commented code that shows which additional mappings are needed,

Extend the commented code that shows which additional mappings are needed,
or which regions need to be reserved.  As it turns out, a region we should
not map is the PCIe address space.  Making a PCIe device try to do DMA to
an address in PCIe address space will obviously not make its way to SMMU
and host memory.  We'll probably have to add an API for that.