media: dt-bindings: i2c: document OV4689

Add device-tree binding documentation for OV4689 image sensor driver,
and the relevant MAINTAINERS entries.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Mikhail Rudenko <mike.rudenko@gmail.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>