dmaengine: documentation to the new callback mechanism

Adding documentation in dmaengine/provider.txt on the usage of callback
with result, callback_result, function pointer to retrieve transanction
result from a DMA submission.

Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Reviewed-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>