x86: acpi: Pack global NVS into ACPI table

Now that platform-specific ACPI global NVS is added, pack it into
ACPI table and get its address fixed up.

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>