iommu/exynos: Rename variables to reflect their purpose

This patch renames some variables to make the code easier to understand.
'domain' is replaced by 'iommu_domain' (more generic entity) and really
meaningless 'priv' by 'domain' to reflect its purpose.

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>