x86: doc: Add note about the debug FSP usage on BayTrail

The debug FSP image is bigger in size than the normal FSP image. This
patch adds a small description on how to use this FSP debug version
by changing CONFIG_FSP_ADDR.

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