camera: v4l2: Make ISPIF a v4l2 subdev.

This makes it easier to control stream on/off using ISPIF
instead of CAMIF. RDI streaming/snapshot also becomes a
possibility.

Note that this change still turns on PIX0 streaming right
after ISPIF configuration. This is because ISPIF streamon
needs to happen before sensor streamon, and right now sensor
stream is turned on before configuring VFE pipeline.

Signed-off-by: Shuzhen Wang <shuzhenw@codeaurora.org>