watchdog: dw_wdt: Convert to use watchdog infrastructure

Convert driver to use watchdog infrastructure. This includes
infrastructure support to handle watchdog keepalive if the watchdog
is running while the watchdog device is closed.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Tested-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>