mmc: MMC_MTK should depend on HAS_DMA

If NO_DMA=y:

    ERROR: "dma_alloc_coherent" [drivers/mmc/host/mtk-sd.ko] undefined!
    ERROR: "dma_unmap_sg" [drivers/mmc/host/mtk-sd.ko] undefined!
    ERROR: "dma_map_sg" [drivers/mmc/host/mtk-sd.ko] undefined!
    ERROR: "dma_free_coherent" [drivers/mmc/host/mtk-sd.ko] undefined!

Add a dependency on HAS_DMA to fix this.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>