arm: make it possible to disable the SMMU driver

Introduce a Kconfig option for the ARM SMMUv1 and SMMUv2 driver.

Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>
Acked-by: Julien Grall <julien.grall@arm.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
CC: jbeulich@suse.com

---
Changes in v3:
- rename SMMUv2 to ARM_SMMU
- improve help message
- use if ARM

Changes in v2:
- rename HAS_SMMUv2 to SMMUv2
- move SMMUv2 to xen/drivers/passthrough/Kconfig

xen/drivers/passthrough/Kconfig

@@ -1,3 +1,15 @@
 
 config HAS_PASSTHROUGH
 	bool
+
+if ARM
+config ARM_SMMU
+	bool "ARM SMMUv1 and v2 driver"
+	default y
+	---help---
+	  Support for implementations of the ARM System MMU architecture
+	  versions 1 and 2.
+
+	  Say Y here if your SoC includes an IOMMU device implementing the
+	  ARM SMMU architecture.
+endif

xen/drivers/passthrough/arm/Makefile

@@ -1,2 +1,2 @@
 obj-y += iommu.o
-obj-y += smmu.o
+obj-$(ARM_SMMU) += smmu.o