iommu/mediatek: Include liunx/dma-mapping.h

This fixes a compile error when cross-compiling the driver
on x86-32.

Signed-off-by: Joerg Roedel <jroedel@suse.de>
Link: https://lore.kernel.org/r/20200713101648.32056-1-joro@8bytes.org

drivers/iommu/mtk_iommu.h

@@ -15,6 +15,7 @@
 #include <linux/iommu.h>
 #include <linux/list.h>
 #include <linux/spinlock.h>
+#include <linux/dma-mapping.h>
 #include <soc/mediatek/smi.h>
 
 #define MTK_LARB_COM_MAX	8