Implement full support for the watchdog in i.MX systems.
Pretimeout support is optional because the watchdog hardware
on i.MX31 does not support pretimeouts.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Message-id: 20200517162135.110364-3-linux@roeck-us.net
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
[PMM: added Property array terminator entry]
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>