AMD IOMMU v2: minor cleanup

Despite this array living in an __init function, having such be an
automatic variable is rather inefficient in terms of generated code.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/drivers/passthrough/amd/iommu_detect.c
+++ b/xen/drivers/passthrough/amd/iommu_detect.c
@@ -66,7 +66,7 @@ void __init get_iommu_features(struct am
 {
     u32 low, high;
     int i = 0 ;
-    char * feature_str[] = {
+    static const char *__initdata feature_str[] = {
         "- Prefetch Pages Command", 
         "- Peripheral Page Service Request", 
         "- X2APIC Supported",