This adds a device tree binding documentation for OV7720/OV7725 sensor.

Cc: Jacopo Mondi <jacopo+renesas@jmondi.org>
Cc: Hans Verkuil <hans.verkuil@cisco.com>
Cc: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Cc: Rob Herring <robh+dt@kernel.org>
Reviewed-by: NRob Herring <robh@kernel.org>
Reviewed-by: NJacopo Mondi <jacopo@jmondi.org>
Reviewed-by: NLaurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: NAkinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: NSakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: NMauro Carvalho Chehab <mchehab+samsung@kernel.org>