dt-bindings: extcon: siliconmitus,sm5502-muic: document connector

Document recently added connector for SM5502 MUIC:

  qcom/msm8916-samsung-serranove.dtb: extcon@14: 'connector' does not match any of the regexes: 'pinctrl-[0-9]

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>