dt-bindings: media: nxp,imx-mipi-csi2: Mark clock-frequency as deprecated

dt-bindings: media: nxp,imx-mipi-csi2: Mark clock-frequency as deprecated

Usage of the clock-frequency property, which is already optional, is
discouraged in favour of using assigned-clock-rates (and
assigned-clock-parents where needed). Mark the property as deprecated,
and update the examples accordingly.

Link: https://lore.kernel.org/r/20250822002734.23516-11-laurent.pinchart@ideasonboard.com
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Hans Verkuil <hverkuil+cisco@kernel.org>