Revert "FROMLIST: iommu: Export core IOMMU functions to kernel modules"

This reverts commit 82047fdd.

Superseded by arm-smmu changes posted upstream.

Change-Id: If4c2c29195ed2350803c5ca2b4d63d9a84682ed8
Signed-off-by: Will Deacon <willdeacon@google.com>