watchdog: cdns: Add comment for expire_now function

IP itself has no reg/no bit which can be used for this functionality.
Add this note to the driver to make sure that none will be asking for
that. Current method is to setup 1s timeout and hang() which is done via
wdt_expire_now().

Signed-off-by: Michal Simek <michal.simek@xilinx.com>

drivers/watchdog/cdns_wdt.c

@@ -251,6 +251,7 @@ static const struct wdt_ops cdns_wdt_ops = {
 	.start = cdns_wdt_start,
 	.reset = cdns_wdt_reset,
 	.stop = cdns_wdt_stop,
+	/* There is no bit/reg/support in IP for expire_now functionality */
 };
 
 static const struct udevice_id cdns_wdt_ids[] = {