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 <[e-mail]>
Acked-by: Steven Rostedt (Google) <[e-mail]>
Acked-by: Masami Hiramatsu <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

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

···
 s390*)
   ARG1=%r2
 ;;
+mips*)
+  ARG1=%r4
+;;
 *)
   echo "Please implement other architecture here"
   exit_untested