watchdog: ts72xx_wdt: convert driver to watchdog core

Cleanup this driver and convert it to use the watchdog framework API.
Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Mika Westerberg <mika.westerberg@iki.fi>
[groeck: Dropped initialization of static variable]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>