driver: base: cpu: export device_online/offline

And the {lock,unlock}_device_hotplug so that they can be used from
modules.

This is in preparation to replace cpu_up/down with
device_online/offline; which kernel/torture.c will require to be
exported to be built as a module.

Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Qais Yousef <qais.yousef@arm.com>