watchdog: max77620: Add comment to clarify set_timeout procedure

Clarify why we need to ping the watchdog before changing the timeout by
quoting the MAX77714 datasheet.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Lee Jones <lee.jones@linaro.org>