media: rkisp1: Split CSI handling to separate file

Not all ISP instances include a MIPI CSI-2 receiver. To prepare for
making it optional, move code related to the CSI-2 receiver to a
separate file.

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>