media: ov9650: add device tree binding

Now the ov9650 driver supports device tree probing. So this adds a
device tree binding documentation.

Cc: Sylwester Nawrocki <s.nawrocki@samsung.com>
Cc: Jacopo Mondi <jacopo@jmondi.org>
Cc: H. Nikolaus Schaller <hns@goldelico.com>
Cc: Hugues Fruchet <hugues.fruchet@st.com>
Cc: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Reviewed-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>