x86: apl: Add the term "Interrupt Timer Subsystem" to ITSS files

ITSS stands for "Interrupt Timer Subsystem", so add that term to the
description of the relevant files.

Signed-off-by: Wolfgang Wallner <wolfgang.wallner@br-automation.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>

arch/x86/cpu/apollolake/itss.c

@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: GPL-2.0
 /*
- * Something to do with Interrupts, but I don't know what ITSS stands for
+ * Interrupt Timer Subsystem
  *
  * Copyright (C) 2017 Intel Corporation.
  * Copyright (C) 2017 Siemens AG

arch/x86/include/asm/arch-apollolake/itss.h

@@ -1,5 +1,7 @@
 /* SPDX-License-Identifier: GPL-2.0 */
 /*
+ * Interrupt Timer Subsystem
+ *
  * Copyright (C) 2017 Intel Corporation.
  * Copyright 2019 Google LLC
  *