[media] ov7640: add new ov7640 driver

This based on the wis-ov7640.c driver that's part of the go7007 driver.
It has been converted to a v4l subdev driver by Pete Eberlein, and I made
additional cleanups.
Based on work by: Pete Eberlein <pete@sensoray.com>

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>