media: v4l: Add 14-bit raw greyscale pixel format

The new format is called V4L2_PIX_FMT_Y14. Like V4L2_PIX_FMT_Y10 and
V4L2_PIX_FMT_Y12 it is stored in two bytes per pixel but has only two
unused bits at the top.

Signed-off-by: Daniel Glöckner <dg@emlix.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>