With ACPI Spec 6.5 chapter 5.2.12.20, each processor in LoongArch
systems has a Core Programmable Interrupt Controller in MADT. The
value of its type is 0x11 in the spec and defined as enum variable
ACPI_MADT_TYPE_CORE_PIC in the Linux kernel.

Physical IDs can be retrieved from MADT for LoongArch systems during
initialization and they can be retrieved from the _MAT output for
hotplug CPUs.

Add physical CPU ID enumeration for LoongArch systems.
Signed-off-by: Bibo Mao <maobibo@loongson.cn>
[ rjw: Subject and changelog edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>