iommu/exynos: Select ARM_DMA_USE_IOMMU

For IOMMU to use on Exynos platforms, we need to enable ARM_DMA_USE_IOMMU. It
would be better to select it by default when EXYNOS_IOMMU is enabled.

Signed-off-by: Tushar Behera <tushar.b@samsung.com>
Reviewed-by: Sachin Kamat <sachin.kamat@samsung.com>
Reviewed-by: Cho KyongHo <pullip.cho@samsung.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>