shim: avoid building of vendor IOMMU code

There's no use for IOMMU code in the shim. Disable at least the vendor-
specific code, until eventually IOMMU code can be disabled altogether.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>

xen/arch/x86/configs/pvshim_defconfig

@@ -22,5 +22,7 @@ CONFIG_EXPERT=y
 # CONFIG_LIVEPATCH is not set
 # CONFIG_SUPPRESS_DUPLICATE_SYMBOL_WARNINGS is not set
 # CONFIG_TRACEBUFFER is not set
+# CONFIG_AMD_IOMMU is not set
+# CONFIG_INTEL_IOMMU is not set
 # CONFIG_DEBUG is not set
 # CONFIG_GDBSX is not set