media: dt-bindings: media: ov5647: Document pwdn-gpios

Document in dt-schema bindings for the ov5647 sensor the optional
'pwdn-gpios' property.

Signed-off-by: Jacopo Mondi <jacopo@jmondi.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>