ata: select DW_DMAC in case of SATA_DWC

Since sata_dwc_460ex.c was moved to generic DMA driver we have to ensure that
user can still compile it.

Fixes: 8b34448 (sata_dwc_460ex: move to generic DMA driver)
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

drivers/ata/Kconfig

@@ -270,6 +270,7 @@ config ATA_PIIX
 config SATA_DWC
 	tristate "DesignWare Cores SATA support"
 	depends on 460EX
+	select DW_DMAC
 	help
 	  This option enables support for the on-chip SATA controller of the
 	  AppliedMicro processor 460EX.