x86: Document ACPI S3 support

Now that we have ACPI S3 support on Intel MinnowMax board, document
some generic information of S3 and how to test it.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Stefan Roese <sr@denx.de>