media: dt-bindings: mt9m114: Add slew-rate DT-binding

The MT9M114 supports the different slew rates (0 to 7) on the output pads.
At the moment, this is hardcoded to 7 (the fastest rate).
The user might want to change this values due to EMC requirements.

Add the 'slew-rate' property to the MT9M114 DT-bindings for selecting
the desired slew rate.

Signed-off-by: Mathis Foerst <mathis.foerst@mt.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>