dt-bindings: i2c: convert i2c-cros-ec-tunnel to json-schema

Convert the google,cros-ec-i2c-tunnel binding to YAML and add it as a
property of google,cros-ec.yaml.

Reviewed-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: Ricardo Cañuelo <ricardo.canuelo@collabora.com>
Link: https://lore.kernel.org/r/20201021114308.25485-2-ricardo.canuelo@collabora.com
[robh: add ref to i2c-controller.yaml]
Signed-off-by: Rob Herring <robh@kernel.org>