watchdog: xen_wdt: use the watchdog subsystem

Change the xen_wdt driver to use the watchdog subsystem instead of
registering and manipulating the char device directly through the misc
API. This is mainly getting rid of the "write" and "ioctl" methods and
part of the watchdog control logic (which are all implemented by the
watchdog subsystem).

Even though the watchdog subsystem supports registering and handling
multiple watchdog devices at the same time, the xen_wdt driver has an
inherent limitation of only one device due to the way the Xen hypervisor
exposes watchdog functionality. However, the driver can now coexist with
other watchdog devices (supported by different drivers).

Signed-off-by: Radu Rendec <rrendec@arista.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>