x86: acpi: Add CSRT description

Add CSRT [1] description as it provided in Linux kernel.

[1]: http://www.uefi.org/sites/default/files/resources/CSRT%20v2.pdf



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