ACPI: processor: idle: Rearrange declarations in header file

Group all of the declarations of functions that belong to the ACPI
processor idle driver together in one place in processor.h.

While at it, drop the unnecessary extern modifier from the declaraions
of two functions.

Signed-off-by: Huisong Li <lihuisong@huawei.com>
Link: https://patch.msgid.link/20250911112408.1668431-3-lihuisong@huawei.com


[ rjw: Subject and changelog rewrite ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>