ACPI: Split out ACPI PSS from ACPI Processor driver

The ACPI processor driver is currently tied too closely
to the ACPI P-states (PSS) and other related constructs
for controlling CPU performance.

The newer ACPI specification (v5.1 onwards) introduces
alternative methods to PSS. These new mechanisms are
described within each ACPI Processor object and so they
need to be scanned whenever a new Processor object is detected.
This patch introduces a new Kconfig symbol to allow for
finer configurability among the two options for controlling
performance states. There is no change in functionality and
the option is auto-selected by the architectures which support it.

A future commit will introduce support for CPPC: A newer method of
controlling CPU performance. The OS is not expected to support
CPPC and PSS at the same time, so the Kconfig option lets us make
the two mutually exclusive at compile time.

Signed-off-by: Ashwin Chaugule <ashwin.chaugule@linaro.org>
[ rjw: Changelog ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>