driver core: cpu: make cpu_subsys const

Now that the driver core can properly handle constant struct bus_type,
move the cpu_subsys variable to be a constant structure as well, placing
it into read-only memory which can not be modified at runtime.

Cc: "Rafael J. Wysocki" <rafael@kernel.org>
Link: https://lore.kernel.org/r/2024010548-crane-snooze-a871@gregkh


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>