platform/chrome: cros_ec: Add a flag to track registration state

platform/chrome: cros_ec: Add a flag to track registration state

Introduce a `registered` flag to the `struct cros_ec_device` to allow
callers to determine if the device has been fully registered and is
ready for use.

This is a preparatory step to prevent race conditions where other drivers
might try to access the device before it is fully registered or after
it has been unregistered.

Link: https://lore.kernel.org/r/20250828083601.856083-5-tzungbi@kernel.org
Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>