watchdog: f71808e_wdt: refactor to platform device/driver pair

Driver so far wasn't ported to the driver model and registered
the watchdog device out of the init after probing the I/O ports
for a watchdog with correct vendor and device revision.

Keep the device detection part at init time, but move watchdog
registration to a platform driver probe function.

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/9e1088839662e5c357286cab0b9de0bb0602e4fd.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>