dmaengine: Add mailing list address to the documentation

It's not the first time I've got a private email in regard to
dmatest.c module.

Motivate people, by adding a note, to send their questions
to the mailing list instead

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>