dts: yaml: remove unused id field

The 'id' field was never used and tended to just have the compat of the
node.  Lets remove it and removed some code in extract_dts_includes.py
related to it.  Added a warning if 'id' is set in a yaml so we can
remove it going forward.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>