x86/api: Rename mp_register_lapic in a comment

Commit 31d2092eb0c23636b73d2c24c0c11b66470cef58 ("x86: move
mp_register_lapic_address to boot.c") renamed mp_register_lapic
to acpi_register_lapic. But mp_register_lapic remains in a
comment. So the patch rename it.

Signed-off-by: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>
Cc: Len Brown <lenb@kernel.org>
Link: http://lkml.kernel.org/r/50625239.3050403@jp.fujitsu.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>