media: v4l: Add 1X16 16-bit greyscale media bus code definition

This extends the greyscale media bus family originally from
MEDIA_BUS_FMT_Y8_1X8 up to MEDIA_BUS_FMT_Y14_1X14 by adding
MEDIA_BUS_FMT_Y16_1X16, and behaves the same way with 16 bits.
Add its documentation in subdev-formats.rst

Signed-off-by: Benjamin Mugnier <benjamin.mugnier@foss.st.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>