x86: fsp: Update the FSP API with the end-firmware method

This new method is intended to be called when UEFI shuts down the 'boot
services', i.e. any lingering code in the boot loader that might be used
by the OS.

Add a definition for this new method and update the comments a little.

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