sys: device_mmio: polish doxygen

() Add marks to denote code blocks so they are rendered correctly
   on generated pages.
() Fix misplaced in,out in @param. If any parameters are
   output, mark the others as input for clarity.
() Also move the previously non-visible comment about the file
   into the doxyge group description.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>