coresight: docs: Create common sub-directory for coresight trace.

There are two files in the Documentation/trace directory relating to
coresight, with more to follow, so create a Documentation/trace/coresight
directory and move existing files there. Fixup index to reference
new location.

Update MAINTAINERS to reference this sub-directory rather than the
individual files.

Signed-off-by: Mike Leach <mike.leach@linaro.org>
Reviewed-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>