media: v4l: subdev: Add pre_streamon and post_streamoff callbacks

Add pre_streamon and post_streamoff callbacks that can be used to set a
CSI-2 transmitter to LP-11 or LP-111 mode. This can be used by receiver
drivers to reliably initialise the receiver when its initialisation
requires software involvement.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>