watchdog: lantiq: Convert to watchdog_device

Instead of doing the ioctl handling manually just use register a
watchdog_device and let the watchdog framework do the ioctl handling.

This also removes the ltq_wdt_bootstatus_set typedef and replaces it
with a structure providing the chip specific functions pointer.
The watchdog_init_timeout() function is now used and the initial timeout
can be provided in device tree.
If the watchdog was already activated it will not be stopped any more,
but the settings from the driver will be used and the watchdog subsystem
will take care.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>