media: dt-bindings: ov8856: Document YAML bindings

This patch adds documentation of device tree in YAML schema for the
OV8856 CMOS image sensor.

Signed-off-by: Dongchun Zhu <dongchun.zhu@mediatek.com>
Signed-off-by: Robert Foss <robert.foss@linaro.org>
Reviewed-by: Maxime Ripard <mripard@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>