ACPI / processor: Initialize per_cpu(processors, pr->id) properly

Commit ac212b69 (ACPI / processor: Use common hotplug infrastructure)
forgot about initializing the per-CPU 'processors' variables which
lead to ACPI cpuidle failure to use C-states and caused boot slowdown
on multi-CPU machines.

Fix the problem by adding per_cpu(processors, pr->id) initialization
to acpi_processor_add() and add make acpi_processor_remove() clean it
up as appropriate.

Also modify acpi_processor_stop() so that it doesn't clear
per_cpu(processors, pr->id) on processor driver removal which would
then cause problems to happen when the driver is loaded again.

This version of the patch contains fixes from Yinghai Lu.

Reported-and-tested-by: Yinghai Lu <yinghai@kernel.org>
Reported-and-tested-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>