iommu: restrict scratch page quarantining to x86

All IOMMU_QUARANTINE_* options are exposed on Arm since 163c6b5898
("xen/arm: pci: introduce PCI_PASSTHROUGH Kconfig option"). However,
only NONE and BASIC are implemented in any Arm iommu driver since
63919fc4d1 ("xen/arm: smmuv3: Add PCI devices support for SMMUv3") and
ca8f6ffeb6 ("xen/arm: smmuv2: Add PCI devices support for SMMUv2").
SCRATCH_PAGE is not yet implemented in any Arm iommu driver. Restrict
scratch page quarantining to x86 for now.

Reported-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Stewart Hildebrand <stewart.hildebrand@amd.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>

xen/drivers/passthrough/Kconfig

@@ -90,4 +90,5 @@ choice
 		bool "basic"
 	config IOMMU_QUARANTINE_SCRATCH_PAGE
 		bool "scratch page"
+		depends on X86
 endchoice