x86: apic_*.c: add description to AMD's extended LVT functions

Signed-off-by: Robert Richter <robert.richter@amd.com>
Cc: oprofile-list <oprofile-list@lists.sourceforge.net>
Cc: Barry Kasindorf <barry.kasindorf@amd.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>