ata: sata_dwc_460ex: select only core part of DMA driver

There is no need to have a platform driver compiled since the DMA driver is
used as a library.

Tested-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Tejun Heo <tj@kernel.org>