AMD IOMMU: struct amd_iommu remove padding on 64 bit

Remove 16 bytes of padding from struct amd_iommu on 64bit builds
reducing its size to 120 bytes, allowing it to span one fewer
cachelines.

Signed-off-by: Richard Kennedy <richard@rsk.demon.co.uk>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>