media: mt9t031: deprecate, move to staging

This driver is unused and depends on the deprecated soc-camera framework.
Move it to staging in preparation for being removed unless someone does
the work to convert it to a proper V4L2 subdev driver.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>