ARM: bL_switcher: filter CPU hotplug requests when the switcher is active

Trying to support both the switcher and CPU hotplug at the same time
is tricky due to ambiguous semantics.  So let's at least prevent users
from messing around with those logical CPUs the switcher has removed
and those which were not active when the switcher was activated.

Signed-off-by: Nicolas Pitre <nico@linaro.org>