ACPI: Decouple ACPI idle and ACPI processor drivers

This patch introduces a new Kconfig symbol, ACPI_PROCESSOR_IDLE,
which is auto selected by architectures which support the ACPI
based C states for CPU Idle management.

The processor_idle driver in its present form contains declarations
specific to X86 and IA64. Since there are no reasonable defaults
for other architectures e.g. ARM64, the driver is selected only for
X86 or IA64.

This helps in decoupling the ACPI processor_driver from the ACPI
processor_idle driver which is useful for the upcoming alternative
patchwork for controlling CPU Performance (CPPC) and CPU Idle (LPI).

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