ACPI / processor: Make it possible to get local x2apic id via _MAT

Logical processors with APIC ID values of 255 and greater are
required to have a Processor Device object and must convey the
processor's APIC information to OSPM using the Processor Local
X2APIC structure, but not until ACPI 5.1, X2APIC structure was
not supported in _MAT method.

_MAT is needed for CPU hotplug and system with more than 255
CPUs will definitely need X2APIC structure, so add its support
in map_mat_entry() to make it possible to get local x2apic id
via _MAT based on ACPI 5.1.

Signed-off-by: Hanjun Guo <hanjun.guo@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>