arm: mxs: Correct CONFIG_SPL_NO_CPU_SUPPORT option

At present this is defined in Kconfig but there is a separate one in the
CONFIG whitelist. It looks like these are duplicates.

Rename the non-Kconfig one and remove it from the whitelist.

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