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>

diff --cc Documentation/devicetree/bindings/watchdog/gpio-wdt.txt
Simple merge

diff --cc drivers/watchdog/Kconfig
Simple merge

diff --cc drivers/watchdog/Makefile
Simple merge

diff --cc drivers/watchdog/gpio_wdt.c
Simple merge