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/lib/Kconfig b/lib/Kconfig
index 5fe5776..2f69085 100644
--- a/lib/Kconfig
+++ b/lib/Kconfig
@@ -429,6 +429,9 @@
 	bool
 	default n
 
+config IOMMU_HELPER
+	bool
+
 config DMA_DIRECT_OPS
 	bool
 	depends on HAS_DMA && (!64BIT || ARCH_DMA_ADDR_T_64BIT)