iommu/mtk: Drop IOVA cookie management

The core code bakes its own cookies now.

CC: Yong Wu <yong.wu@mediatek.com>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/b856648e7ee2b1017e7c7c02e2ddd50eaf72cbf7.1628682048.git.robin.murphy@arm.com


Signed-off-by: Joerg Roedel <jroedel@suse.de>