iommu-helper: move the IOMMU_HELPER config symbol to lib/

This way we have one central definition of it, and user can select it as
needed.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Anshuman Khandual <khandual@linux.vnet.ibm.com>
diff --git a/arch/s390/Kconfig b/arch/s390/Kconfig
index aa734f3..89c0c72 100644
--- a/arch/s390/Kconfig
+++ b/arch/s390/Kconfig
@@ -708,7 +708,9 @@
 menuconfig PCI
 	bool "PCI support"
 	select PCI_MSI
+	select IOMMU_HELPER
 	select IOMMU_SUPPORT
+
 	help
 	  Enable PCI support.
 
@@ -732,9 +734,6 @@
 config HAS_IOMEM
 	def_bool PCI
 
-config IOMMU_HELPER
-	def_bool PCI
-
 config NEED_SG_DMA_LENGTH
 	def_bool PCI