[media] v4l2-subdev: allow subdev to send an event to the v4l2_device notify function

All drivers use custom notifications, in particular when source changes.
The bridge only has to map the subdev that sends it to whatever video node it is connected to.

Signed-off-by: Jean-Michel Hautbois <[e-mail]>
Signed-off-by: Mauro Carvalho Chehab <[e-mail]>

Documentation/video4linux/v4l2-framework.txt

···
 An example on how the V4L2 events may be used can be found in the OMAP
 3 ISP driver (drivers/media/platform/omap3isp).
 
+A subdev can directly send an event to the v4l2_device notify function with
+V4L2_DEVICE_NOTIFY_EVENT. This allows the bridge to map the subdev that sends
+the event to the video node(s) associated with the subdev that need to be
+informed about such an event.
 
 V4L2 clocks
 -----------

include/media/v4l2-subdev.h

···
 #define V4L2_SUBDEV_IR_TX_NOTIFY		_IOW('v', 1, u32)
 #define V4L2_SUBDEV_IR_TX_FIFO_SERVICE_REQ	0x00000001
 
+#define	V4L2_DEVICE_NOTIFY_EVENT		_IOW('v', 2, struct v4l2_event)
+
 struct v4l2_device;
 struct v4l2_ctrl_handler;
 struct v4l2_event_subscription;