x86: fix section mismatch warning in topology.c:arch_register_cpu

arch_register_cpu() is only defined for HOTPLUG_CPU code
so simple fix is to ignore references by annotating the
function __ref.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>