media: dt-bindings: media: imx258: add bindings for IMX258 sensor

Add bindings for the IMX258 camera sensor.  The bindings, just like the
driver, are quite limited, e.g. do not support regulator supplies.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>