iommu/rockchip: Use IOMMU device for dma mapping operations

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>