iommu/exynos: Refactor code (no direct register access)

This patch changes some internal functions to have access to the state of
sysmmu device instead of having only it's registers. This will make the
code ready for future extensions.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>