dmaengine: add slave-dma maintainer

Slave-dma has become the predominant usage model for dmaengine and needs
special attention.  Memory-to-memory dma usage cases will continue to be
maintained by Dan.

Cc: Alan Cox <alan@linux.intel.com>
Acked-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>