ACPI / processor: use container_of instead of casting first structure member

Use container_of instead of casting first structure member to resolve
acpi_madt_local_apic/sapic/x2apic from acpi_subtable_header.

Signed-off-by: Fabian Frederick <fabf@skynet.be>
[ rjw: Subject and changelog ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>