select IOMMU_API when DMAR and/or AMD_IOMMU is selected

These two IOMMUs can implement the current version of this API. So
select the API if one or both of these IOMMU drivers is selected.

Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>

drivers/base/Makefile

@@ -11,6 +11,7 @@ obj-$(CONFIG_FW_LOADER)	+= firmware_class.o
 obj-$(CONFIG_NUMA)	+= node.o
 obj-$(CONFIG_MEMORY_HOTPLUG_SPARSE) += memory.o
 obj-$(CONFIG_SMP)	+= topology.o
+obj-$(CONFIG_IOMMU_API) += iommu.o
 ifeq ($(CONFIG_SYSFS),y)
 obj-$(CONFIG_MODULES)	+= module.o
 endif