kdb: add usage string of 'per_cpu' command

Signed-off-by: Namhyung Kim <namhyung@gmail.com>
Signed-off-by: Jason Wessel <jason.wessel@windriver.com>

kernel/debug/kdb/kdb_main.c

@@ -2892,7 +2892,7 @@ static void __init kdb_inittab(void)
 	  "Send a signal to a process", 0, KDB_REPEAT_NONE);
 	kdb_register_repeat("summary", kdb_summary, "",
 	  "Summarize the system", 4, KDB_REPEAT_NONE);
-	kdb_register_repeat("per_cpu", kdb_per_cpu, "",
+	kdb_register_repeat("per_cpu", kdb_per_cpu, "<sym> [<bytes>] [<cpu>]",
 	  "Display per_cpu variables", 3, KDB_REPEAT_NONE);
 	kdb_register_repeat("grephelp", kdb_grep_help, "",
 	  "Display help on | grep", 0, KDB_REPEAT_NONE);