MFC r262606, r262607, r262608, r262609, r262610, r269528, r269609, r271133:

 - Move imported dts source from sys/contrib/dts/ to sys/gnu/dts.
 - Fix some missing properties.
 - Import dts-related header files.
 - Update everything to latest vendor branch representing 3.17-rc2
   level of Linux DTS API.
MFC 271140:

  Delete old arm dts tree. This was created by cherry picking from a
  full vendor tree. This worked great until it was time to update, but
  now it is time to update. Hit the rest button by removing this branch
  and re-adding it by a full copy of whatever is in the vendor tree.
MFC r271143, r271151:  Reimport dts from vendor branch after proper trim.
MFC r262626, r262626, r264979, r265154, r265162, r265163, r267319, r270863,
    r270991, r271045:

  Enhancements related to compiling FDT data...

 - Use proper include path for dtc as well as cpp.
 - Call cpp with -P to avoid printing line markings.
 - Allow FDT_DTS_FILE to be a list, either in the makedtb target, or in a
   kernel config file.
 - Add the dts include directory to the -I list when doing arm builds.
 - Invoke make_dtb with MACHINE defined for cross building friendliness.
 - Allow the make_dtb script to work outside of a "make buildkernel"
   context by setting MACHINE from uname -m if it's not set already.
 - Use sh -c '...' to launch the dtb build scripts with env prepended,
   otherwise it tries to treat the env var stuff as a script file name.