iommu/rockchip: Prepare to support generic DMA mapping

Set geometry for allocated domains and fix .domain_alloc() callback to
work with IOMMU_DOMAIN_DMA domain type, which is used for implicit
domains on ARM64.

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>