selftests/ftrace: add mips support for kprobe args string tests

This is the mips variant of commit <3990b5baf225> ("selftests/ftrace:
Add s390 support for kprobe args tests").

Signed-off-by: Ze Zhang <zhangze@loongson.cn>
Acked-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

tools/testing/selftests/ftrace/test.d/kprobe/kprobe_args_string.tc

@@ -25,6 +25,9 @@ ppc*)
 s390*)
   ARG1=%r2
 ;;
+mips*)
+  ARG1=%r4
+;;
 *)
   echo "Please implement other architecture here"
   exit_untested