watchdog: Add a device managed API for watchdog_register_device()

This helps in reducing code in .remove callbacks and sometimes
dropping .remove callbacks entirely.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>