dmaengine: dw: Remove unused internal property

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

The rest two cases, i.e. Intel Quark and PPC460ex, instantiate DMA driver and
use its channels exclusively for hardware, which means there is no available
channel for any other purposes anyway.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Vinod Koul <vkoul@kernel.org>