x86: mp_init: Switch parameter names in start_aps()

These parameters are named differently from elsewhere in this file. Switch
them to avoid confusion.

Also add comments to this function.

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