dt-bindings: extcon: Add support for cros-ec device

This patch add documentation for binding of USB Type C cable detection
mechanism is using EXTCON subsystem. The device can detect the presence
of display out but it may also detect other external accessories when
external accessories is attached or detached.

Signed-off-by: Benson Leung <bleung@chromium.org>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Reviewed-by: Chanwoo Choi <cw00.choi@samsung.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>