iommu: Make the iova library a module

The iova library has use outside the intel-iommu driver, thus make it a
module.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
diff --git a/drivers/iommu/Kconfig b/drivers/iommu/Kconfig
index f1fb1d3..0a141cd 100644
--- a/drivers/iommu/Kconfig
+++ b/drivers/iommu/Kconfig
@@ -42,7 +42,7 @@
 endmenu
 
 config IOMMU_IOVA
-	bool
+	tristate
 
 config OF_IOMMU
        def_bool y