watchdog: f71808e_wdt: migrate to new kernel watchdog API

Migrating the driver lets us drop the watchdog misc device boilerplate
and reduces size by 285 lines. It also brings us support for new
functionality like CONFIG_WATCHDOG_HANDLE_BOOT_ENABLED.

This incurs a slight backwards-compatibility break, because the new
kernel watchdog API doesn't support unloading modules for drivers
whose watchdog hardware is reported to be running.

This means following scenario will be no longer supported:
 - BIOS has enabled watchdog
 - Module is loaded and unloaded without opening watchdog
 - module_exit is expected to succeed and disable watchdog HW

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/35d9dbf57b58c5f003cef31dc256ec2fec044524.1628525954.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>