[media] s5p-csis: Enable v4l subdev device node

Set v4l2_subdev flags for a host driver to create a sub-device
node for the driver so the subdev can be directly configured
by applications. Add the subdev open() handler.

Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>