iommu/rockchip: Use DMA API to manage coherency

Use DMA API instead of architecture internal functions like
__cpuc_flush_dcache_area() etc.

The biggest difficulty here is that dma_map and _sync calls require some
struct device, while there is no real 1:1 relation between an IOMMU
domain and some device. To overcome this, a simple platform device is
registered for each allocated IOMMU domain.

With this patch, this driver can be used on both ARM and ARM64
platforms, such as RK3288 and RK3399 respectively.
Signed-off-by: Shunqian Zheng <zhengsq@rock-chips.com>
Signed-off-by: Tomasz Figa <tfiga@chromium.org>
Signed-off-by: Joerg Roedel <jroedel@suse.de>