docs(debug mode): add D-mode to modes_with_access

Author: neverlandiz

lib/arch_obj_models/csr.rb

@@ -361,6 +361,8 @@ def modes_with_access
       ["M", "S", "U", "VS", "VU"]
     when "VS"
       ["M", "S", "VS"]
+    when "D"
+      ["M", "D"]
     else
       raise "unexpected priv mode"
     end