media: v4l: ctrls: Add a control for HDR mode

Add V4L2_CID_HDR_MODE as a menu item control to set the HDR mode of the
sensor, and its documentation.
Menu items are not standardized as they differ for each sensors.

Signed-off-by: Benjamin Mugnier <benjamin.mugnier@foss.st.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>