dmaengine: dw: append MODULE_ALIAS for platform driver

The commit 9cade1a4 (dma: dw: split driver to library part and platform
code) introduced a separate platform driver but missed to add a
MODULE_ALIAS("platform:dw_dmac"); to that module.

The patch adds this to get driver loaded automatically if platform device is
registered.

Reported-by: "Blin, Jerome" <jerome.blin@intel.com>
Fixes: 9cade1a4 (dma: dw: split driver to library part and platform code)
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>