x86: itss: Add a Kconfig option to enable/disable ITSS driver

Add a Kconfig option to support enabling/disabling the inclusion of
the ITSS driver depending on the platform.

Atuomatically select the ITSS driver when building for Apollo Lake.

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: squashed in http://patchwork.ozlabs.org/patch/1232761/

]
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>