iommu/mediatek-v1: Support COMPILE_TEST

Add COMPILE_TEST support to Mediatek v1 so I have less chance of
breaking it again in future. This just needs the ARM dma-iommu API
stubbing out for now.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/a8d7c0b4393b360ce556f5f15e229d1e2fe73c83.1741702556.git.robin.murphy@arm.com


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