r309531:
  Implement fake pmap_mapdev_attr() for ARMv6.  This function is referenced,
  but never called from DRM2 code. Also, real behavior of pmap_mapdev_attr()
  in ARM world is unclear as we don't have any additional attribute for a
  device memory type.
r309553:
  Fix build breakage caused by r309531.
r309604:
  Fix the armv6 build after r309553.