iommu/mediatek: Convert M4Uv2 to iommu_fwspec

Our per-device data consists of the M4U instance and firmware-provided
list of LARB IDs, which is a perfect fit for the generic iommu_fwspec
machinery. Use that directly as a simpler alternative to the custom
archdata code.

CC: Yong Wu <yong.wu@mediatek.com>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Tested-by: Yong Wu <yong.wu@mediatek.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>