dmaengine: imx: fix the build failure on x86_64

commit 6bd08127 "dmaengine: imx-dma: merge old dma-v1.c with
imx-dma.c" removed the dependency in config for the imx dma driver,
whereas it should depend on ARCH_MXS

Signed-off-by: Vinod Koul <vinod.koul@linux.intel.com>