iommu/arm-smmu: fix compilation issue when !CONFIG_ARM_AMBA

If !CONFIG_ARM_AMBA, we shouldn't try to register ourselves with the
amba_bustype.

Signed-off-by: Will Deacon <will.deacon@arm.com>
diff --git a/drivers/iommu/arm-smmu.c b/drivers/iommu/arm-smmu.c
index 0ae4dd3..6fe7922 100644
--- a/drivers/iommu/arm-smmu.c
+++ b/drivers/iommu/arm-smmu.c
@@ -2004,8 +2004,10 @@
 	if (!iommu_present(&platform_bus_type))
 		bus_set_iommu(&platform_bus_type, &arm_smmu_ops);
 
+#ifdef CONFIG_ARM_AMBA
 	if (!iommu_present(&amba_bustype))
 		bus_set_iommu(&amba_bustype, &arm_smmu_ops);
+#endif
 
 	return 0;
 }