dmaengine: Add note to dmatest documentation about supported channels

The documentation is not so clear for newbies in a sense of what type of the
channels are supported by it.

Clarify this by adding a note at the preamble of the documentation.

Reported-by: "Zhu, Tony" <tony.zhu@intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-By: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>