media: rkisp1: Make the internal CSI-2 receiver optional

Not all ISP versions integrate a MIPI CSI-2 receiver.

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>