x86: Allow devices to write an SSDT

Call the new core function to write the SSDT. This is made up of fragments
generated by devices that have the fill_ssdt() method.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Wolfgang Wallner <wolfgang.wallner@br-automation.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>