watchdog: gpio: Convert to use GPIO descriptors

This converts the GPIO watchdog driver to use GPIO descriptors
instead of relying on the old method to read out GPIO numbers
from the device tree and then using those with the old GPIO
API.

The descriptor API keeps track of whether the line is active
low so we can remove all active low handling and rely on the
GPIO descriptor to deal with this for us.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>