iommu/exynos: Remove custom platform device registration code

Commit 09515ef5 ("of/acpi: Configure dma operations at probe time for
platform/amba/pci bus devices") postponed the moment of attaching IOMMU
controller to its device, so there is no need to register IOMMU controllers
very early, before all other devices in the system. This change gives us an
opportunity to use standard platform device registration method also for
Exynos SYSMMU controllers.

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