x86: baytrail: acpi: Hide internal UART per GNVS setting

If global NVS says internal UART is not enabled, hide it in the ASL
code so that OS won't see it.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: George McCollister <george.mccollister@gmail.com>
Tested-by: George McCollister <george.mccollister@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>