iommu/exynos: Rework iommu group initialization

This patch replaces custom code in add_device implementation with
iommu_group_get_for_dev() call and provides the needed callback.

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