iommu/rockchip: Perform a reset on shutdown

Trying to do a kexec whilst the iommus are still on is proving to be
a challenging exercise. It is terribly unsafe, as we're reusing the
memory allocated for the page tables, leading to a likely crash.

Let's implement a shutdown method that will at least try to stop
DMA from going crazy behind our back. Note that we need to be
extra cautious when doing so, as the IOMMU may not be clocked
if controlled by a another master, as typical on Rockchip system.

Suggested-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>