iommu/exynos: use exynos-iommu specific typedef

This commit introduces sysmmu_pte_t for page table entries and
sysmmu_iova_t vor I/O virtual address that is manipulated by
exynos-iommu driver. The purpose of the typedef is to remove
dependencies to the driver code from the change of CPU architecture
from 32 bit to 64 bit.

Signed-off-by: Cho KyongHo <pullip.cho@samsung.com>