dmaengine: DW DMAC: enable memory-to-memory transfers support

All known devices, which use DT for configuration, support
memory-to-memory transfers. So enable it by default, if we read
configuration from DT.

Acked-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>