power: reset: sc27xx: Change to use cpu_down()

To allow the SC27XX driver can be built as a module, and the
freeze_secondary_cpus() symbol is not exported, thus we can change
to use the exported cpu_down() API to shut down other cpus to avoid
racing, which is same as the freeze_secondary_cpus().

Signed-off-by: Baolin Wang <baolin.wang7@gmail.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>