ACPI / processor_idle: replace PREFIX with pr_fmt

Like few of the other ACPI modules, replace PREFIX with pr_fmt and
change all the printk call sites to use pr_* companion functions
in processor_idle.

Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>