ACPI / PNP: constify device IDs

Instead of re-creating the array on the stack each time
is_cmos_rtc_device() gets called, make the array 'static const'.

Signed-off-by: Mathias Krause <minipli@googlemail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>