ARM: 8063/1: bL_switcher: fix individual online status reporting of removed CPUs

The content of /sys/devices/system/cpu/cpu*/online  is still 1 for those
CPUs that the switcher has removed even though the global state in
/sys/devices/system/cpu/online is updated correctly.

It turns out that commit 0902a904 ("Driver core: Use generic
offline/online for CPU offline/online") has changed the way those files
retrieve their content by relying on on the generic attribute handling
code.  The switcher, by calling cpu_down() directly, bypasses this
handling and the attribute value doesn't get updated.

Fix this by calling device_offline()/device_online() instead.

Signed-off-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>