x86: Move itss.h from Apollo Lake to the generic x86 include directory

The code in this file is not specific to Apollo Lake. According to
coreboot sources (where this code comes from), it is common to at least:
  * Apollo Lake
  * Cannon Lake
  * Ice Lake
  * Skylake

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>
3 files changed