dt-bindings: display: add google, cros-ec-anx7688.yaml

ChromeOS EC ANX7688 is a display bridge that converts HDMI 2.0 to
DisplayPort 1.3 Ultra-HDi (4096x2160p60). It is an Analogix ANX7688 chip
which is connected to and operated by the ChromeOS Embedded Controller
(See google,cros-ec.yaml). It is accessed using I2C tunneling through
the EC and therefore its node should be a child of an EC I2C tunnel node
(See google,cros-ec-i2c-tunnel.yaml).

ChromOS EC ANX7688 is found on Acer Chromebook R13 (elm)

Signed-off-by: Dafna Hirschfeld <dafna.hirschfeld@collabora.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Robert Foss <robert.foss@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210518141927.24795-2-dafna.hirschfeld@collabora.com