dt-bindings: Add cros-ec Type C port driver

Some Chrome OS devices with Embedded Controllers (EC) can read and
modify Type C port state.

Add an entry in the DT Bindings documentation that lists out the logical
device and describes the relevant port information, to be used by the
corresponding driver.

Signed-off-by: Prashant Malani <pmalani@chromium.org>
Reviewed-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Reviewed-by: Benson Leung <bleung@chromium.org>
Signed-off-by: Rob Herring <robh@kernel.org>