dt-bindings: extcon: maxim,max77843: restrict connector properties

Do not allow any other properties in connector child, except what
usb-connector.yaml evaluates.

Fixes: 9729cad0

 ("dt-bindings: extcon: maxim,max77843: Add MAX77843 bindings")
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>