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

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
diff --git a/arch/mips/kernel/scall64-o32.S b/arch/mips/kernel/scall64-o32.S
index 049a9c8..b0541dd 100644
--- a/arch/mips/kernel/scall64-o32.S
+++ b/arch/mips/kernel/scall64-o32.S
@@ -522,7 +522,7 @@
 	PTR	sys_ioprio_get			/* 4315 */
 	PTR	compat_sys_utimensat
 	PTR	compat_sys_signalfd
-	PTR	sys_ni_syscall
+	PTR	sys_ni_syscall			/* was timerfd */
 	PTR	sys_eventfd
 	PTR	sys32_fallocate			/* 4320 */
 	PTR	sys_timerfd_create