iommu/sprd: Release dma buffer to avoid memory leak

When attaching to a domain, the driver would alloc a DMA buffer which
is used to store address mapping table, and it need to be released
when the IOMMU domain is freed.

Signed-off-by: Chunyan Zhang <chunyan.zhang@unisoc.com>
Link: https://lore.kernel.org/r/20230331033124.864691-2-zhang.lyra@gmail.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>