iommu/exynos: Use struct exynos_iommu_domain in internal structures

Replace all remaining usage of struct iommu_domain with struct
exynos_iommu_domain in all internal structures and functions.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Tested-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Signed-off-by: Joerg Roedel <jroedel@suse.de>