MIPS: Document former use of timerfd(2) syscall number.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
diff --git a/arch/mips/kernel/scall32-o32.S b/arch/mips/kernel/scall32-o32.S
index 7f5468b..7f1377e 100644
--- a/arch/mips/kernel/scall32-o32.S
+++ b/arch/mips/kernel/scall32-o32.S
@@ -565,7 +565,7 @@
 	sys	sys_ioprio_get		2	/* 4315 */
 	sys	sys_utimensat		4
 	sys	sys_signalfd		3
-	sys	sys_ni_syscall		0
+	sys	sys_ni_syscall		0	/* was timerfd */
 	sys	sys_eventfd		1
 	sys	sys_fallocate		6	/* 4320 */
 	sys	sys_timerfd_create	2