x86: use actual CPU number for allocating memory

Use actual CPU number, instead of maximum cpu configured, to allocate
stack memory in 'load_sipi_vector'

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