x86/mpparse: Use new APIC registration function

Aside of switching over to the new interface, record the number of
registered CPUs locally, which allows to make num_processors and
disabled_cpus confined to the topology code.

No functional change intended.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Michael Kelley <mhklinux@outlook.com>
Tested-by: Sohil Mehta <sohil.mehta@intel.com>
Link: https://lore.kernel.org/r/20240213210251.830955273@linutronix.de