ARM: bL_switcher: hot-unplug half of the available CPUs

In a regular kernel configuration, all the CPUs are initially available.
But the switcher execution model uses half of them at any time.  Instead
of hacking the DTB to remove half of the CPUs, let's remove them at
run time and make sure we still have a working switcher configuration.
This way, the same DTB can be used whether or not the switcher is used.

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