dt-bindings: hdmi-connector: add DDC I2C bus phandle documentation

Add an optional ddc-i2c-bus phandle property that points to
an I2C master controller that handles the connector DDC pins.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Acked-by: Rob Herring <robh@kernel.org>