watchdog: GPIO-controlled watchdog

This patch adds a watchdog driver for devices controlled through GPIO,
(Analog Devices ADM706, Maxim MAX823, National NE555 etc).

Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>