luci-mod-system: add LED interval parameter

As documented

https://openwrt.org/docs/guide-user/base-system/led_configuration

Signed-off-by: Paul Donald <newtwen+github@gmail.com>

modules/luci-mod-system/htdocs/luci-static/resources/view/system/leds.js

@@ -125,6 +125,9 @@ return view.extend({
 			}
 		};
 
+		o = s.option(form.Value, 'interval', _('Interval'), _('milliseconds'));
+		o.placeholder = '50';
+
 		return m.render();
 	}
 });