The choice is between looping over the physical range and performing
single cache line operations, or to map highmem pages somewhere, as
cache range ops are possible only on virtual addresses.

Because L2 range ops are much faster, we go with the later by factoring
the physical-to-virtual address conversion and use a fixmap entry for it
in the HIGHMEM case.

Possible future optimizations to avoid the pte setup cost:

 - do the pte setup for highmem pages only

 - determine a threshold for doing a line-by-line processing on physical
   addresses when the range is small
Signed-off-by: Nicolas Pitre <nico@marvell.com>