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

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

The mediatek iommu driver relied on an implicit include of dma-mapping.h,
but for some reason that is no longer there in 4.12-rc1:

drivers/iommu/mtk_iommu_v1.c: In function 'mtk_iommu_domain_finalise':
drivers/iommu/mtk_iommu_v1.c:233:16: error: implicit declaration of function 'dma_zalloc_coherent'; did you mean 'debug_dma_alloc_coherent'? [-Werror=implicit-function-declaration]
drivers/iommu/mtk_iommu_v1.c: In function 'mtk_iommu_domain_free':
drivers/iommu/mtk_iommu_v1.c:265:2: error: implicit declaration of function 'dma_free_coherent'; did you mean 'debug_dma_free_coherent'? [-Werror=implicit-function-declaration]

This adds an explicit #include to make it build again.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 208480bb27 ('iommu: Remove trace-events include from iommu.h')
Signed-off-by: Joerg Roedel <jroedel@suse.de>