ACPI: Allow selection of the ACPI processor driver for ARM64

Now that the ACPI processor driver has been decoupled from
the C states and P states functionality, make it selectable on
ARM64 so that it can be used by others e.g. CPPC.

The C states and P states code is selected only on X86 or
IA64 until the relevant support is added on ARM64.

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