media: i2c: Add driver for THine THP7312

The THP7312 is an external camera ISP from THine. Add a V4L2 subdev
driver for it.

Signed-off-by: Paul Elder <paul.elder@ideasonboard.com>
Co-developed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
[Sakari Ailus: squash a patch to fix missing mutex_unlock by Laurent.]
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>