dt-bindings: chrome: google,cros-ec-typec: restrict allowed properties

Describe exactly what properties are allowed in Google Chrome OS EC Type
C port, so the schema can properly validate the DTS.  Existing DTS
defines always connectors with unit addresses, not a sole "connector"
child.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Prashant Malani <pmalani@chromium.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220811062245.4316-1-krzysztof.kozlowski@linaro.org