iommu/exynos: Rework runtime PM links management

add_device is a bit more suitable for establishing runtime PM links than
the xlate callback. This change also makes it possible to implement proper
cleanup - in remove_device callback.

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