linux-imx

Documentation: add Linux tracing to Sphinx TOC tree

This just add a index.rst for trace subsystem. More docs will
be added later.

Cc: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Changbin Du <changbin.du@intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>