dma: move dw_dmac driver to an own directory

The dw_dmac driver is going to be split into multiple files. To make this more
convenient move it to an own directory.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>