On most resets, a "reset reason: unknown" would clutter the console,
because the watchdog doesn't support differentiating between POR and
RST. Reduce the clutter by only printing the message when we have
something interesting to say (i.e. watchdog reset).

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>