ACPI / processor: Make it possible to get APIC ID via GIC

Introduce a new function map_gic_id() to allow APIC IDs to be obtained
from the GIC Structure introduced by ACPI 5.

Signed-off-by: Hanjun Guo <hanjun.guo@linaro.org>
[rjw: Subject and changelog]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>