dt-bindings: imx: ldb: Add ddc-i2c-bus property

Document the ddc-i2c-bus property used by imx-ldb driver to read EDID
information via I2C interface.

Signed-off-by: Akshay Bhat <akshay.bhat@timesys.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>