[media] tw2804: add support for the Techwell tw2804

This is based on the wis-tw2804.c driver that's part of the go7007 driver.
It has been converted to a v4l subdev driver by Volokh Konstantin, and I
made additional cleanups.
Based on work by: Volokh Konstantin <volokh84@gmail.com>

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