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: NWill Deacon <will.deacon@arm.com>

drivers/iommu/arm-smmu.c

@@ -2004,8 +2004,10 @@ static int __init arm_smmu_init(void)
 	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;
 }