The Interrupt Timer Subsystem (ITSS) is not specific to Apollo Lake, so
move it to a common location within arch/x86.

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>
[bmeng: conditionally build itss.c]
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>