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>