x86: acpi: Introduce a stub to generate CSRT

Here is a stub function that generates an empty CSRT. If the target platform
provides acpi_fill_csrt() function, it will be used to populate the table.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>