iommu: Allow building iova.c independently

In preparation for sharing the IOVA allocator, split it out under its
own Kconfig symbol.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>
diff --git a/drivers/iommu/Kconfig b/drivers/iommu/Kconfig
index 325188e..a839ca9 100644
--- a/drivers/iommu/Kconfig
+++ b/drivers/iommu/Kconfig
@@ -13,6 +13,9 @@
 
 if IOMMU_SUPPORT
 
+config IOMMU_IOVA
+	bool
+
 config OF_IOMMU
        def_bool y
        depends on OF && IOMMU_API
@@ -91,6 +94,7 @@
 	bool "Support for Intel IOMMU using DMA Remapping Devices"
 	depends on PCI_MSI && ACPI && (X86 || IA64_GENERIC)
 	select IOMMU_API
+	select IOMMU_IOVA
 	select DMAR_TABLE
 	help
 	  DMA remapping (DMAR) devices support enables independent address