distinguish between UC and WB memory so that we can map the page to
either a region 6 address (for UC) or a region 7 address (for WB).
This change is only now possible, because previously we would map
regions 6 and 7 with 256MB translations and on top of that had the
kernel mapped in region 7 using a wired translation. The introduction
of the PBVM moved the kernel into its own region and freed up region
7 and allowed us to revert to standard page-sized translations.
This commit inroduces pmap_page_to_va() that respects the attribute.
01e4705d93