The watchdog on these SoCs is enabled by default on system boot, so a
driver is especially useful. According to data sheet the mode register
containing the timeout can be configured only once, but I couldn't
verify this on the sama5d2. Regardless, the driver takes care not to
change the mode register unless necessary. Implementation that want to
leave to the OS the decision which timeout to choose, can just keep
pinging with the POR-default of 16 seconds and the OS will be able to
set the final timeout.

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