The watchdog driver will remain unprobed if the driver or the OF node
is disabled, but the reset reason is useful even then.

System reset and reset source determination is achieved with the RCC
peripheral for which we have a reset controller driver.
Move the code over there, so reset reason and reset are available
always.

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