MFC r257648, r257649, r257660:

  Begin reducing code duplication in arm pmap.c and pmap-v6.c by factoring
  out common code related to mapping device memory into a new devmap.c file.

  Remove the growing duplication of code that used pmap_devmap_find_pa() and
  then did some math with the returned results to generate a virtual address,
  and likewise in reverse to get a physical address.  Now there are a pair
  of functions, arm_devmap_vtop() and arm_devmap_ptov(), to do that.  The
  bus_space_map() implementations are rewritten in terms of these.

  Move remaining code and data related to static device mapping into the
  new devmap.[ch] files.  Emphasize the MD nature of these things by using
  the prefix arm_devmap_ on the function and type names (already a few of
  these things found their way into MI code, hopefully it will be harder to
  do by accident in the future).
MFC r257669, r257672, r257673, r257676, r257678:

  Call initarm_lastaddr() later in the init sequence, after establishing
  static device mappings, rather than as the first of the initializations
  that a platform can hook into.  This allows a platform to allocate KVA
  from the top of the address space downwards for things like static device
  mapping, and return the final "last usable address" result after that and
  other early init work is done.

  Because some platforms were doing work in initarm_lastaddr() that needs to
  be done early, add a new initarm_early_init() routine and move the early
  init code to that routine on those platforms.

  Make PTE_DEVICE a synonym for PTE_NOCACHE on armv4, to make it easier to
  share the same code on both architectures.

  Add new helper routines for arm static device mapping.  The new code
  allocates kva space from the top down for the device mappings and builds
  entries in an internal table which is automatically used later by
  arm_devmap_bootstrap().  The platform code just calls the new
  arm_devmap_add_entry() function as many times as it needs to (up to 32
  entries allowed; most platforms use 2 or 3 at most).

  Remove imx local devmap code and use the essentially identical common
  code that got moved from imx_machdep.c to arm/devmap.c.
MFC r257924:

  Apparently with "const uint32_t foo = 0x60;" gcc doesn't consider 'foo'
  to be a constant integer suitable for use in a case label, so use #defines.