watchdog: Add ChromeOS EC-based watchdog driver

Embedded Controller (EC) present on Chromebook devices
can be used as a watchdog.
Implement a driver to support it.

Signed-off-by: Lukasz Majczak <lma@chromium.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20240126095721.782782-3-lma@chromium.org
Signed-off-by: Lee Jones <lee@kernel.org>