mfd: cros_ec: Register EC-based watchdog subdevice

Add ChromeOS EC-based watchdog as EC subdevice.

Signed-off-by: Lukasz Majczak <lma@chromium.org>
Link: https://lore.kernel.org/r/20240119084328.3135503-4-lma@chromium.org


Signed-off-by: Lee Jones <lee@kernel.org>