x86: amd_iommu: move to drivers/iommu/

This should ease finding similarities with different platforms,
with the intention of solving problems once in a generic framework
which everyone can use.

Compile-tested on x86_64.

Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>