smp-urgent-2021-09-12

 - Remove DEFINE_SMP_CALL_CACHE_FUNCTION() which is a left over of the
   original hotplug code and now causing trouble with the ARM64 cache
   topology setup due to the pointless SMP function call. It's not longer
   required as the hotplug callbacks are guaranteed to be invoked on the
   upcoming CPU.

 - Remove the deprecated and now unused CPU hotplug functions

 - Rewrite the CPU hotplug API documentation