docs(debug mode): add D mode to multi_xlen_in_mode

Author: neverlandiz

lib/cfg_arch.rb

@@ -142,6 +142,8 @@ def multi_xlen_in_mode?(mode)
       else
         raise "Unexpected configuration state"
       end
+    when "D"
+      mxlen.nil?
     else
       raise ArgumentError, "Bad mode"
     end