[media] ov2640: convert from soc-camera to a standard subdev sensor driver

Convert ov2640 to a standard subdev driver. The soc-camera driver no longer
uses this driver, so it can safely be converted.

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