watchdog: f71808e_wdt: dynamically allocate watchdog driver data

While the driver will only match against a single device, convention is
to dynamically allocate the driver data.

Suggested-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/1baffdc410d6476cccffe9712cec56350335812a.1628525954.git-series.a.fatoum@pengutronix.de


Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>