Use the first registered IOMMU device for dma mapping operations, and
drop the domain platform device.

This is similar to exynos iommu driver.
Signed-off-by: Jeffy Chen <jeffy.chen@rock-chips.com>
Reviewed-by: Tomasz Figa <tfiga@chromium.org>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>