media: rkisp1: csi: Implement a V4L2 subdev for the CSI receiver

The CSI receiver is a component separate from the ISP or the resizers.
It is actually optional, not all device model include a CSI receiver. On
some SoCs CSI-2 support can be provided through an external CSI-2
receiver, connected to the ISP's parallel input.

To support those use cases, create a V4L2 subdev to model the CSI
receiver. It will allow the driver to handle both internal and external
CSI receivers the same way.

The next commit will plumb the CSI subdev to the rest of the driver,
replacing direct function calls.

Signed-off-by: Paul Elder <paul.elder@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Dafna Hirschfeld <dafna@fastmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>