media: dt-bindings: media: ov772x: Document endpoint props

Document endpoint properties for the parallel bus type and
add them to the example.

Specify a few constraints:
- If the bus type is BT.656 no hsync or vsync polarities can be
  specified.
- If the bus width is 10 bits, not data-shift can be applied.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Signed-off-by: Jacopo Mondi <jacopo+renesas@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>