cpuidle / ACPI : move cpuidle_device field out of the acpi_processor_power structure

Currently we have the cpuidle_device field in the acpi_processor_power structure.
This adds a dependency between processor.h and cpuidle.h

Although it is not a real problem, removing this dependency has the benefit of
separating a bit more the cpuidle code from the rest of the acpi code.
Also, the compilation should be a bit improved because we do no longer
include cpuidle.h in processor.h. The preprocessor was generating 30418 loc
and with this patch it generates 30256 loc for processor_thermal.c, a file
which is not concerned at all by cpuidle, like processor_perflib.c and
processor_throttling.c.

That may sound ridiculous, but "small streams make big rivers" :P

This patch moves this field into a static global per cpu variable like what is
done in the intel_idle driver.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>