x86: apl: Adjust FSP-M code to avoid hard-coded address

Update this code to calculate the address to use, rather than hard-coding
it. Obtain the requested stack size from the FSP.

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>