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

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/kernel/scall64-64.S

@@ -404,7 +404,7 @@ sys_call_table:
 	PTR	sys_ioprio_get
 	PTR	sys_utimensat			/* 5275 */
 	PTR	sys_signalfd
-	PTR	sys_ni_syscall
+	PTR	sys_ni_syscall			/* was timerfd */
 	PTR	sys_eventfd
 	PTR	sys_fallocate
 	PTR	sys_timerfd_create		/* 5280 */