iommu/exynos: Make use of iommu_device_register interface

Register Exynos IOMMUs to the IOMMU core and make them
visible in sysfs. This patch does not add the links between
IOMMUs and translated devices yet.

Cc: Marek Szyprowski <m.szyprowski@samsung.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-samsung-soc@vger.kernel.org
Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>