media: v4l: ctrls: add a control for flash/strobe duration

media: v4l: ctrls: add a control for flash/strobe duration

Add a V4L2_CID_FLASH_DURATION control to set the duration of a
flash/strobe pulse. This controls the length of the flash/strobe pulse
output by device (typically a camera sensor) and connected to the flash
controller. This is different to the V4L2_CID_FLASH_TIMEOUT control,
which is implemented by the flash controller and defines a limit after
which the flash is "forcefully" turned off again.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Richard Leitner <richard.leitner@linux.dev>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil+cisco@kernel.org>