Documentation: dmaengine: Add a description of what dmatest does

Currently it can difficult to determine what dmatest does without
reading the source code. Let's add a description.

The description is taken mostly from the patch header of
commit 4a776f0a ("dmatest: Simple DMA memcpy test client").
It has been edited and updated slightly. Nevertheless the new text was
largely written by Haarvard Skinnemoen and was copied from another
patch, already committed to the kernel, which has Haarvard's SoB:
attached to it.

Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20211118100952.27268-2-daniel.thompson@linaro.org


Signed-off-by: Vinod Koul <vkoul@kernel.org>