iommu: make inclusion of amd directory conditional

Nothing in there is active if CONFIG_AMD_IOMMU is not enabled, so the whole
directory can depend on that switch as well.

Fixes: cbe94c6e ("iommu/amd: Move Kconfig and Makefile bits down into amd directory")
Signed-off-by: Rolf Eike Beer <eb@emlix.com>
Reviewed-by: Lu Baolu <baolu.lu@linux.intel.com>
Link: https://lore.kernel.org/r/1894970.atdPhlSkOF@devpool92.emlix.com


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